Adding toolboxes to the sides

If going that route, I'd get a 350 or larger. A 250 is too light duty a unit for carrying that much extra weight and the 7.3 is a low output motor too. Reeliable but low output.

Yea I'm thinking 350 now, I just really don't want a dually as we're headed to central America and the roads are narrow. . However, if that's what I need, that's what I'll get. The driving is slow down there especially past Mexico. The roads are terrible.

Buying one already with a flatbed or service body would probably be cheaper and less work.

I had 10' service body and not only floor was 7" above the frame, but I had to add 4x4 planks to clear sides with camper.
Having basement camper that made for 13-6 total height. Not too comfortable and costing extra fuel to push that frontal area.
Flatbeds add 4-5" to the height.
Lot of variables, so carry tape measure with you checking it out.
On the other hand, the original Cab & Chassis on 2006 F350 had stronger suspension than regural bed trucks.
I had no problem carrying the heavy body and 6000 lb camper set 2 ft behind the cabin.

I'll suggest that a service body is HEAVY. Staying light and minimizing overall vehicle height while maximizing ground clearance for a a setup that is basically an expedition vehicle is what you should be honing in on, and you're on the right track.
Of course buying already setup flatbed possibly with a couple tool boxes should be more economical than building it. Think ranching areas for buying a used flatbed. MT, WY, eastern wa, anywhere rural out west has a lot of flatbed trucks.
Idk how mechanically inclined you are, but on one hand, staying simple with a 7.3 Powerstroke or a 12V or 24v Cummins is IMO the best for mechanical and electrical simplicity, but the older the truck gets the more "stuff" gets wore out.

I'd be looking for a lighter weight aluminum sided hard side TC, flatbed preferably aluminum but those are more expensive and the tool boxes, if not already setup, I'd wait til you have the camper and flatbed setup and then match the boxes. You can get fairly economical generic side boxes for not too much $.

Expedition to South America, while I've never been there, I'd think srw truck would be largely preferable. Get a 250 or 350 (2500 or 3500), doesn't matter. You'll need to upgrade the rear suspension anyway and the rest of the truck is the same regardless of model and since you're looking at old trucks, miles and condition is far more important than the badge on the fenders. Those are only ther to provide something to debate on a camper forum! Lol
While I'd be tempted to get 19.5 tires and rims for capacity and durability, they are extremely rough riding and hard on suspension components on the front of a vehicle that isn't heavy enough to need them, on unimproved roads. And you lose the ability to air them down compared to E load tires. Ideally, 19.5s or some big super singles in the back and normal tires up front, but that creates a mismatch and the need for 2 types of spares. This I'd do some serious research on and shoe the truck with what's available south of the equator in case you shred a tire or 2.

Good luck with your build and trip. BTW, if you're in the PNW currently, there's a basically brand new 7.3 Powerstroke for sale on Craigslist Seattle. But they want $27k for it.

I definitely world prefer one already setup with the flatbed or tool box. Yea 13.6 feet is really high, he'll my fifth wheel is 12.5ft. A basement sounds nice though, lol. The thing is I have to get my wife, kid future kid, and dog in this thing hence why I can't go for an 1000lb camper. I need something in the middle on size.

I'm willing to travel to find what I need but I'm not going to pay 27k for a truck that's at least 20 years old, does it come with the camper?. I saw a real nice one on expedition portal for 50k with the camper of course, still expensive to me. More like 10k for an old 7.3. I got my current 7.3(not 4x4 unfortunately though) for 6.5k from a guy that owned a large truck shop and just had the transmission done and the truck repainted the original color. It was immaculate according to my mechanic. I'm thinking about trying to get a truck when I'm down in the south again, they seem a lot cheaper there and I've got a mechanic I trust. I've driven my current one with only one breakdown(on our first trip water pump went out and destroyed some stuff) and we've be from, Jacksonville to Seattle, Seattle to Tucson by Yellowstone, Tucson to Baja and back, and now are on our way to Alaska( we're in Prince George, BC right now) , all with our 35ft fifth wheel plus my wife uses it as her daily driver when she works about half the year. I love the older 7.3's even more than newer ones although I have a 99 with 310k on it.

The tires is definitely something I have to look into because they just don't carry a lot of the same ones here.

That's good advice on waiting on getting the stuff to setup the camper. Why do you say to get a hard sided camper? I could get a pop up and it would be lighter and shorter. Just a thought, I want a hardsided one, just wanted to hear your opinion.

Thanks for all the info and if those huge expedition vehicles that all seem to be from Germany, they drive all the way around the world, including Latin America, I'm sure I can do it with a truck camper. I really want a srw but those things have to be as wide as a drw and are definitely 13ft+ and they make it. If I wasn't cheap, I might buy one actually, lol.

The thing is you can do the Pan American in a semi if you want, that's what it was built for. It's when you want to get off of it that you have problems. It's like us boondocking for months at a time in our 35ft fifth wheel when we could have a 4x4 Tacoma and a 800lb TC. We just are a lot more limited on where we can go. I don't plan to off road and people do the Pan American in 2wd cars. All that is to say, others do it and if they can I can.

I'd like a cheap truck camper so we can stay in hostels or hotels when we want to without feeling bad about it but we also want to be comfortable when we are camping. The size of the camper is going to come down to my wife honestly. I generally make all the big decisions but picking out a camper she'll be happy with is very important obviously, especially with two young children. I could live in camper shell or rooftop tent personally, lol.

If you are willing to travel, in California flatbeds and especially service body sell very cheap.
About 4 years ago I sold my 2001 F250 4WD supercab with 7' custom flatbed for $4300 and took me like 6 months to find a buyer.
I made the 7.3l engine bone-dry, but the truck had like 320k miles, still running perfect.
Had 2 of 7.3 and seem new orings on oil cooler + coolant tank have to be replaced every few years.
Other than that (sometimes intercooler boots) really workhorses.
I was thinking about pop-up as 2nd camper for fast trips when I go alone, but for me RV has to have toilet and some water.
Those pop-ups that have them cost more than big campers.
That beside the worry about canvas cracks.

We've been in and out of California for the past three years(best pay in the country is in the central valley), that sounds like my ideal truck, lol. I am not only willing to travel but basically spend my life on the road these days. They really are one of the greatest small diesels ever put in a truck.

I got ya. I want a decent size freshwater tank (the bigger the better) in whatever I get because that's what limits our camping. I'm considering converting it to a composting toilet, heard a lot of good things about them. Otherwise our black tank may become our new problem... The one in our fifth wheel is huge, we could go weeks but can't go more than maybe two weeks at the max without freshwater.

A 48x18x18 steel tool box weighs about 100 lbs. That makes 400 additional pounds taken from your payload. Unless you are going with a DRW or with more expensive aluminum boxes, you will eat up all your payload with none left to carry additional items in those empty boxes.

Well others manage it, I'm sure I can too. Thanks for the advice though. I've seen plenty of truck camper builds with side toolboxes, therefore it's possible. Maybe I should just give up and drop 200k on something already built for me I guess.