Before you do anything, you need to figure out what your truck can actually carry.
Start with checking the published specs for the truck's GVWR, payload, axle and wheel weight capacities. It's very easy to exceed the GVWR and payload numbers, and most trucks going down the highway with slide in campers will be over these numbers, especially SRW 2500s. Remember, payload includes passengers and all cargo. Weighing the truck will get you a good baseline.
The numbers to be careful of are the axle, wheel and tire weight ratings. Exceed these, especially tires, at your peril.
Here's a good site for doing the weight calculations once you figure out the ratings and your baseline weight:
Truck camper weight calculator
Once you figure out what you can realistically carry, you can start looking at campers. Realize that the advertised/brochure weights on campers are almost ALWAYS dry/empty, and with no installed options. To give you an idea, we have a Lance 981, that is advertised as 3600 lbs dry. The actual wet weight of our camper is closer to 4500 than 3600, and 40 gallons of water doesn't weigh 900lbs...
Don't forget to factor in tongue weight if you plan to tow while the camper is mounted. You also may need special hardware for the hitch, depending on the length of the camper and the tongue weight. IMHO Torklift is the best of the bunch for extensions.
You may find you need override springs and a bigger sway bar to fix sag and sway. Lots of folks go with airbags, but I like the simplicity of Torklift Stableloads. Helwig makes big swaybars for most trucks.
Find RV dealers near you and start looking. Even if you don't buy from a dealer, you'll get to look at setups and features and that can help you decide exactly what you want. Used campers in good shape go pretty quickly around here.
If you're looking at $5000 campers, you'll be looking at units that are pretty old. A PPI by a qualified RV tech is probably a good idea, because they can be expensive to fix. Most of it is pretty easy to DIY, if you have the time, but packaging can make things a PITA. The biggest issue with a truck camper is transporting it... as noted above, you'll need to add special equipment to your truck to secure the camper.
We gave things a lot of thought before going with a truck camper, and we've been really happy with our Lance. It's given us way more versatility than a motorhome or camp trailer, and has already provided some great memories.
Some gratuitous pics...