I am a little surprised MiG-29 are given for free. Maybe Belarus will get a good deal on repairing and modernising some those old MiG-29. Currently, Belarus has 41 MiG-29s as per wiki article.  MiG-29 is a light fighter that is aging.  The latest fighter Russians are using is Su-35 which 4++ generation. This is quite an expensive plane. Light fighter MiG-29 will be replaced by Mig-35 which will also cost. But it would not be the main fighter plane in Russian air-force . MiG-29 are still used by many air-forces of the world.

Buk missile systems are also aging. S-300 that Serbia is planning to buy from Belarus to defend its air-space is a superior missile system than Buk.