Personally I wouldn't sweat the 2500/3500 distinction since you're buying used. They're generally identical other than the badge on the fender and a slightly softer rear spring pack (and not even that in all cases). Just make sure the truck you get has adequate axle and tire ratings, and as I mentioned steer clear of 17" tires if you can. If you find that it sags more than you want when you load it just throw on bags or timbrens and call it a day. Getting a good deal on a used truck can be tricky so I personally wouldn't complicate matters by limiting the pool of trucks I was shopping more than necessary.
For your budget I'd be looking for a gas truck, and if you can avoid the popular configurations (namely a crew cab with 4wd) you can get a better deal. If you want a reliable truck I agree that you're going to need to up the budget some.