It sounds like most of the cars being suggested are not going to really save you any money in the long run vs. just keeping your 3.
If you are serious about getting by on the cheap, sell your car outright, and just get some basic transportation, maybe something like an older model Focus. The wagons are very practical and usually have been taken decent care of. A zx5 is also good basic transportation and you'll be able to buy a used one for less than 5k.
If you are trying to buy a car that will compete with your 3 you might as well keep the 3. Buy some tools, and ramps, do the waterpump yourself, and start doing your own maintaining.
Good luck with your house!
06 330i 6mt Spt- Eibach Springs, BMW Perf Susp, Supersprint Exhaust, BSW Audio
94 325i 5mt 4d- M3 springs/susp, Z3 wheels, Chipped, Intake, Exhaust= FUN!