Should I keep this vehicle or what?

I purchased a 2001 Isuzu Rodeo with 118k on it on June 1 from a small car lot. Since my previous truck broke down and I need transportation to work, I had to jump into something QUICKLY! I work an hr away from home. I put 1k down and signed some paperwork and umed the deal was done. Since purchasing the vehicle, the battery has died about 3 times and the truck seems to either be leaking or burning oil excessively. I just got the oil changed last Tuesday and it was nearly bone dry today when I checked it. I also noticed after having the truck for a few days, when looking at the sticker in the window, the oil change was due at 115k but I purchased it with 118k. Where did those 3k miles come from? I read up on Isuzu's and excessive oil usage/leakage seems to be a very common problem. I got a phone call last week from the car lot and they told me that there was a contract that I never signed so that the deal isn't really finalized. I also never paid taxes on the truck and those are due as well. There was a 30 day warranty, but if the contract isn't signed does it apply? The first payment was due on July 1 but I haven't paid it because I'm unsure about this deal. What should I do?