I had to deal with renting (read not renting) before age 25. Good luck, even if you find a company, it will cost a crazy premium. At 18, provided you have a pilot license (with correct certification) you can rent a jet, with same price/requirements. But when you land, still can't rent a car.
I have personal experience driving 2 used Toyota Previa minivans a combined 243,000 miles. The first got rear ended at 300k the second is going strong at 240k currently. First ones still runs, but it was hit hard, and is hard to fix a car with so many miles.
Paid 3k for the first one with 127k miles on it. 2K for the second @ 170k. Both were 1995 S/C models.
Have never had any engine/transmission problems... in fact other than having to replace a $70 rubber bushing (It didn't fail, just was making a slight noise... took about 5 hours tinkering with it) all maintenance has been regular, oil, brake pads, did the spark plugs/wires one time on the first van around 280k.
For your situation I am not sure if it fits the bill, but they are cheap. Have been reliable (unbelievably so in my experience), for the room/utility (We have towed cars, trailers with cars, enclosed trailers, 20 sheets of 4x8 plywood inside, full sound gear plus instruments and 6 people... etc) and it gets decent mpg (22-24). I have set up a bed in the back and driven cross country, or camped in the mountains.. but slept in a nice bed :) The rear seat comes out easy.
A quick look on CL in sfbay and there are several in the 2500 range.
All the other vehicles I have experience with don't really fit your needs.