Where yo buy a Yak is really up to you. Personally I would go to a well known dealer.
As well as the normal things to check when buying any aircraft you should make sure it has had the main spar mod done and the control surfaces re-covered in a 'western' fabric. Also radial engines need a bit more attention to pre-flight checks, operating temperatures etc so if buying from a private seller try and check that he has operated the engine in a professional manner.
The engine TBO question keeps arising and I am not aware of any extensions being given in the UK. It is question that should really be put to the Aviation Authority in the country you wish to register/operate the aircraft.
Our group are very close to needing a new engine and if the CAA were to change the rules just after we had paid out good money on a new one we would be most dis-pleased !!
Regards