I was hell bent on recommending a Subaru given your request for fuel mileage but Thirty Nine has brought a very good option to the table in terms of capability, reliability and frugality go. I too am unsure if you can sleep in one or not but given their cheap entry fee I think it should be something to consider, I know I did. However, eventually my little Subaru won the races. It wasn't in the $3k-5k price bracket at the time but I'm sure you can find an 02-04 in that price range, and with that you get a very safe vehicle, something that was high on my priority list, great mileage considering its AWD, and the ability to sleep in it if needed. I've touched on this before and I'll do so again in this post, but the ide of an "expedition vehicle" is bull****. There are vehicles that get you to where you need to go reliably and there are vehicles that steal our imagination (and wallets). By that I mean if you're truly strapped for cash, get the cheapest and most reliable vehicle you can find like that W123 Benz someone posted, fold the seat down (not sure if that was an option) or remove it altogether and sleep in the back. Seeing the world doesn't require limitless pockets, but it does require you to really consider what is important to you. I'm not at all knocking the guys running several 10's of thousands of dollars worth of equipment on their rig, as that's what makes them happy and that's what car ownership and life in general is all about. But please don't be fooled into thinking that you need to have a sleep platform, front and rear lockers, or even 4wd for that matter. You need a brain, an open mind, and a willingness to make the most out of whatever you're given. I've slept in the back of my Impreza wagon with 2 60+ lb. dogs over 30 nights at rest stops, campsites, hotel parking lots, you name it.
When I decide to sleep, I pull my passenger seat all the way up, put an ammo can, or igloo cooler to fill the void, along with my backpack on top, and that's usually enough to provide an even surface in which to lay my REI sleep pad down. Tossa a pillow and a nice comforter in the mix and I'm in heaven, especially after 10 hours of facetime behind a windshield. Keep in mind that this is done with 2 mangy mutts occupying the other part of my car. There is alot of rearranging of luggage that needs to be done when setting this arrangement up, but my plans of a thule or yakima roof box will hopefully address this issue.
If gas mileage is your main concern, don't compromise on that. Although all the options presented thus far are solid, they also don't offer the greatest mileage, and I wouldn't hesitate to take an old 93 Accord Wagon up to the Artic circle and back with the arrangement I described above if we're truly talking budget vehicle. My list is as follows
1. 85 Benz 300D -
http://phoenix.craigslist.org/evl/cto/4834935414.html
2. 95 Legacy w/EJ22 and 4EAT Auto - EJ22, doesn't share the head gasket issues that the 2.5L EJ25 has in earlier versions + you get Subie AWD, duh. -
http://tucson.craigslist.org/cto/4835195151.html
3. 90 Volvo 240 Wagon - they're tanks and you could sleep in the back and invite 5 friends to join you-
http://phoenix.craigslist.org/wvl/cto/4824388346.html
4. 87 Yota Pickup with cap - This probably should be closer to the top. 2WD single cab 4 banger is going to net you mid-high 20's mpg. Toyota dependability without being beat like a 4WD -http://tucson.craigslist.org/cto/4814340510.html
5 89 4x4 Nissan Pickup - just as solid as a Yota IMHO w/o the pricetag (I owned one so I'm biased) -
http://tucson.craigslist.org/cto/4819378809.html
Runner Up: Tracker -
http://tucson.craigslist.org/cto/4779982313.html
Be realistic in what you need vs what you want. Do you NEED 4x4? Do you need high ground clearance (at the sacrifice of mpg), the list goes on, but at the end of the day the final decision comes down to you , not a forum of people who aren't at all familiar with you and your needs as you are. Writing a list helps, as does being honest with yourself and not letting want's rule the list, especially a list for a "Super Budget Expedition Vehicle".