The massive problem will be insurance, trying to get it covered to drive it back. The polish insurance covers you for 14days after the car is sold (In Poland the car, not the person is insured), unless the seller is leaving the cover on the car and allowing you on his policy, (I know its a strange system), next problem will be a green card, you will be asked to show at the Polish/German border. You or the policy holder will need to get one from the insurers.
If it is a 2000 car I hope you checked for rust (under the arches), the newer cars really have a problem with the 'tin worm',
The fact it has a Polish MOT means nothing, I used to get mine without even going to the centre

, only cost me £15. There is no car tax in Poland.
Just one last thing is the car a Happy End?