Most OP unit would probably be the T70. Not really that OP for it's price, but more the timing of the unit combined with its performance. It could probably be toned down and made cheaper to be more like the Luchs.
The problem would be I guess the overlap between the M3 halftrack craut mower and the cheaper T70. Perphaps the M3 can be adjusted to be more usefull in its default form - perhaps some on the field healing ability which the Soviets lack.
+1
But I think you mean the M5, not the M3 of T1?