I have heard people say mpg's suffer with a 4x4 but I have never noticed it. With trucks that are full time 4x4 (70's and older mostly) this would be an issue but with part time (70's and newer) 4x4's I don't think you would notice a difference. Towing capacity does not increase, in fact as far as towing ratings, 4x4's are lower. When GVWR (gross vehicle weight rating) is figured it is figured as gross vehicle, cargo, passengers etc. and 4x4 equipment must be figured in, usually it's a few hundred pounds difference in what they are rated for. Will it actually make a difference, no. If you need to pull a 6000 lb trailer with a truck that is rated for 6600 GVWR or 6800 GVWR you won't notice a bit if difference, same goes for a 6700 lb. trailer with the same truck. Repairs aren't neccesarily more costly for anything else on the truck, but you do have to maintain the 4x4 components. You'll already be repacking bearings on the front end so that won't make a difference but there are u joints in the axles (unless there are CV joints) and u joints in the driveshaft as well as diff case oil. But if these parts are not used regularly (if 4x4 is rarely used) you won't have to deal with these parts very often, however since they are rarely used they are often times forgotten about and instead of servicing ujoints you end up replacing them, again no big deal. Costs of most 4x4 related repairs really aren't too awful expensive, but that is relative to what you consider too expensive compared to I, how often you use it and your means of justifying it.
In general since you said you are wanting to spend $5000 you'll no doubt be looking at used trucks. It's not hard to find an older 4x2 or 4x4 truck for $5k in good shape, you may have to do some light maint/repair but for 5k you should be able to find a good used truck.
I have owned many 4x4 trucks and I keep them well maintained and don't abuse them too bad and haven't had to rob a bank to buy parts. Alot of times you can find aftermarket stuff at good prices or find a salvage yard and you can get most of what you need, if not try ebay. I would justify it.