I never would have guessed that they started as "cheap" cars when they have so much beautiful and reliable cars now a days. Do you know when they started sending cars here in dubai?
For me, what I do is to rotate it every 3k to 5k just to be safe. I would want to have a flat tire somewhere and to have to replace them there. I am more of a coach potato so I would always go for prevention that takes less time to do that major repairs or replacements.
I will always get the new one. Aside from having a guarantee, it saves me the head ache when I find that there was a problem on the used one that I didn't see during test drive.
Have you checked if there are any fuel leaks? It might also consume more fuel because of the following:
Low tire pressure
lead foot on accelerator
problem with the oxygen sensor.
There might be a problem with the engine. Black smoke can indicate excessive burning of fuel. make sure to check the foloil in the engine to make sure it is not contaminated.
It is possible that the wiring between the AC and the engine got tangled up. As Essah, suggested you might want to check on the electricity first. Does the AC turnoff when you turnoff the engine?
Just as Bhikoo said, a blown head gasket could cause the oil to fill into the radiator. Although there are "do it yourself" ways on how to fix this, it would be better for you to just take it to the mechanic who should have fixed it the first time. Don't forget to ask him for discounts for not doing the job properly the first time.