Very nice car that can last you for a long time.
1. Maintainance is key. If the car is taken care of 150,000 miles out of that engine should not be a problem (make sure the timing belt is changed at 90,000miles)
2. Not sure about years or month, but generally the newer the better.
3. I am not sure about Slovakia, but in the US BMW has a Zero cost maintainance program for the first 50,000 miles so that people bring the cars back to the dealers where they can properly maintain the cars. Helps BMW with resale value (smart).
4. Both trannys are great. My buddy had a 6mt and it was phenominal with a short shift kit. Another friend had one with SMG (lived just outside New York), and said he wouldn't buy another manual given the option to buy SMG/DCT etc.
Good Luck, let us know if you buy one!
"The laws of physics cannot be repealed, even with DSC." - BMW Owner's Manual