Couchsurfing can be a way to find low cost places to sleep, however, you shouldn't view it just as free lodging. Couchsurfing is a community, and typically its recommended to bring your host a gift, take them out to dinner, or something along those lines. If you treat it just as a place to crash, you'll likely find yourself with negative ratings and people unwilling to host you. You should also be getting involved now, as a host, to become established within the community.
I have to ask, how much stuff are you planning to buy on the road? As a solo traveler, you'd have to take a ton of stuff with you and or buy a ton of stuff before you'd fill up a car the size of a civic. I've traveled with a family, with camping gear, in a vehicle that size. Sleeping in such a vehicle wouldn't work well, but beyond that, I can't even guess why you'd need more space.2) Part of my reason for wanting to take a van was to have adequate space for belongings and finds...I could mail stuff home but I would think that would add up quickly. Anyone ever take a Honda Civic or similar car on this kind of trip?
If you only look at the change of value of the vehicle, then 10,000 miles would have little impact. The problem is that there is much more to the cost of buying and selling a car than just the change in value. Typically it costs more to buy a car (retail) than you'll get when you sell it (wholesale). You can reduce that difference somewhat if you only buy and sell on the private party market (i.e. autotrader, craigslist, etc), however even there, there are still other costs of purchasing a car. Any car you're thinking of buying should be taken to a mechanic, especially in a case like this. You could have to pay for a few inspections before you find a suitable vehicle. Then once you do, you'll have title, registration, taxes, and license fees which will add up to at least several hundred dollars.3) I do understand your emphasis on not purchasing a van for a single trip like this, but if I were to find something reliable and affordable, how much might the value of it go down for re-sale when the trip is over?