Best practice bike?

Currently own a couple motorcycles and really want to get a cheap practice bike. Something I can practice sliding, cornering, extreme breaking and pushing personal limits without any fear or care of damaging the bike.

Been thinking seriously about a Z125. Watching various agility courses something like an R3 seems solid but that would easily be double the cost and more power. Supermotos seems like good options too but don’t know a great deal about those.

Any ideas/pros/cons to certain bikes?