rvdreamer18,
Since you asked these questions, I guess I'll take a stab at answering them, with a few personal comments & opinions thrown in for good measure.
1) Is gas powered that much of a negative that we should seek out the old diesel?
Not in my opinion. A diesel chassis usually adds things like air-ride suspensions and such, plus with the engine in the back, it's a much quieter ride up front. They MAY offer a much larger cargo carrying capacity, but not necessarily so.
In my opinion, you DO need to know about chassis weight ratings. Gross Vehicle Weight Rating, Axle Weight Ratings and Gross Combined Weight Ratings. There have been some motorhomes (class A's and C's) manufactured which are over an axle weight rating just loaded with water and fuels. In my opinion, if you wish to travel safely, you need to know and respect the weight ratings of these vehicles. You should try to estimate the MAXIMUM amount of people, food, water and stuff you will be bringing along. With 8-10 people, that estimate will probably be a minimum of about 2,500 lbs.
One thing you should be aware of is that almost anywhere you go, when you're in an RV, you're immediately perceived as "rich" with a lot of disposable income. RV dealership shop hourly rates, from what I've heard, now are in the $150/hour range. Diesel rigs will probably be more expensive to repair than gas rigs, but part of that is because the parts are designed to last longer, so you'll pay for that "privilege" when you replace them. And again, if you have to pay someone to do the work, it WILL be expensive.
2) How many miles is too many on a gas and/or diesel engine?
With proper maintenance, I think most modern gas engines should be good to about 200k miles, and diesel engines to well over 500k, and probably well beyond that. That said, however, parts fail at any time. My estimate is that any major engine repair or replacement on the road would probably be in the $3000-$5000 range at a minimum. Rubber items and such can age out rather than wear out, so they are more a function of time than mileage.
3) How many miles are too few on a gas and/or diesel engine?
Probably doesn't matter gas or diesel in this case. But I'd be wary of any rig which didn't have an average of 3-5,000 miles per year. Now, if it's stored indoors in a climate controlled garage, then maybe less than that would be OK. Again, things can age out.
I'd be more concerned about low generator hours. Most generators that don't get enough use will have carburetor/fuel system problems. Again, a good average might be 50-100 hours per year. Our generator manufacturer (Onan) recommends fairly frequent regular use to keep everything working, and I think most generators are similar in that regard.
4) Any brands to totally avoid?
This will depend upon who you ask. For older vehicles, I think proper & regular maintenance is more important than the manufacturer. Some names of which ones you SHOULD look at are Newmar, Tiffin & Winnebago. If you look at anything on a Prevost chassis, than that would probably be a keeper too! :)
5) Is it smarter to buy from a private seller who maintained it well or a dealer?
People tend to not be fully forthcoming about items when they're trying to sell them. With most dealerships, that's almost a given, though there are some good ones. Private sellers? There are lots of honest people out there and there are a lot of people out there scamming people out of money trying to sell them a non-existent RV. Buyer beware. Always!
6) Will we feel too cramped traveling with that many people in a class C? Though they sleep more, they seem a bit cramped compared to the openness of a class A.
Class A or Class C, you WILL be cramped with 8 people, 10 people would be almost hard to imagine even IF you could find seating for all of them. Again, in my opinion, you have to be careful of weight limits. 8-10 people, with an average weight of 150 lbs/person often used for such calculations implies 1,200-1,500 lbs. of human weight. Add in another 500-700 lbs. for water, plus food, clothing, etc., and you will easily be approaching 3,000 lbs. of needed weight carrying capacity even if you pack VERY lightly. Our rig has about 3,000 lbs. of such capacity and we can make it work with our crew, but our family weight is about 800 lbs. (Ok....a good part of that is me! LOL). If we had to shed 700 pounds of "stuff" to make room for additional passengers, that would be tough to do for anything other than just a day trip.
I'd like to respond to another thing you mentioned. You mentioned putting 2 people on each sofa and the dinette. Well, that is pretty optimistic UNLESS these are pre-teen kids. First of all, with the two sofas, one is usually an RV full or possibly queen fold-out, which can sleep 2 fairly easily, though they're usually not comfortable for adults. Kids? They'll sleep on anything! LOL Also, the opposing one is usually a jack-knife sofa and that can usually only sleep one. Plus, in many rigs, there is not enough aisle width to extend the sofa bed AND the jack-knife at the same time. Ditto, the dinette can probably only sleep two if the people are smaller (again, pre-teen) and well under 5'. In fact, ALL of the sleeping spots from sofas & dinette will probably be shorter than a near 6' adult would find comfortable. If they don't mind sticking their feet over the arm rests, or sleep with their knees bent, it'll work. If they like to stretch out, then they won't be comfortable.
Oh, I should also say that as a rule, the sofas and dinettes can be different lengths/widths in different floorplans, so not all dinettes are the same. Ditto the sofas. Usually, the longer the motorhome, the larger the dinette & sofas will be.
Ok, hope this helps. I still think your best bet is a full size van pulling a bunkhouse travel trailer. I also think you need to start going to local RV dealerships & RV shows and walk around inside. (In our early stages of research, I was looking at floorplans and thinking that a Class B conversion van could work for the 6 of us. Sure looked like it would from what I saw online. Just seeing a real one the first time ended THAT fantasy! LOL) My recommendation is DO NOT talk to any salespeople. Most have never been camping, so they really can't give you much info. A little bit of research and you will easily know more than most RV salespeople. Don't let on that you're even thinking about buying at this point. Just look. Ask US the questions. :)
Good Luck in your search.
~Rick