If you want a four door model, you will have to look at 2007+ models, as 2007 was the first year the four door was produced. Jeeps hold their value pretty well over time. For a 2007 Jeep Wrangler Unlimited (aka JKU - - Unlimited is the four door), the cheapest you are likely to find is $15-17k for a stock vehicle. That's for a base model with no upgrades. You'll still have to do a bit of hunting to find one at that price. For mild to moderate offroading though, you don't really need to upgrade your vehicle with anything big. You don't even have to upgrade at all; the stock Wrangler can go quite a few places. It is meant to be taken offroad, after all. If you want something a step up from a stock vehicle, though, you only need to do a 2" budget boost, maybe go for some slightly bigger tires (31 or 32). You can add a winch for a few hundred bucks and you're all set to do some mild/moderate offroading while still having a decent offroad stance. I recommend taking time to do a bit of research and look around this forum at other people's Jeeps. Lots of pictures around! It will help give you a better idea of what you specifically want your Jeep to have. Look at some stock 2007 JKU's for sale. Go for a couple test drives if you haven't already. Most Jeepers prefer to buy a stock vehicle so they can upgrade it the way they want it.
Welcome to the forum! Look forward to seeing your "look at my new Jeep!" post soon.