230 will be a Komp, 320 a V6. If going auto the 230 will be a 5 speed, the 320 is only available as a 7 speed auto from memory.

Both are on cam chain. 230s can do cam chains if not serviced regularly as they tend to stretch.

Tbh there is not alot to look out for really, front bushes and ball joints wear quite heavily on Mercs, the gearbox guide bush can leak on 5 speed autos, exhaust hangers rot and snap, and thats about it really for common problems with CLKs.