No one can tell you how long your car can keep going - not even a mechanic unless he's psychic.
Before you go to DriveTime or Auto Nation, go to a a regular dealer. Some dealers will let you split up your down payment into multiple payments as long as you have income coming in. You can even walk away with a brand new car. The downside is yes, your interest rate is going be higher, but dealers have access to finance companies you don't have access to.
I suggest you find a fuel economy car that's proven to be reliable and make sure it's within your budget. Then go to a dealer and see what deal they can make you. In some cases, it may be even easier to get a new car than a used one. Let Auto Nation and Drive Time be your last resort.
Hope this helps!