If it's based on the TM then it's basically V3 but without the motor cage.
Depending on what you're dropping in there (e.g. polarstar, wolverine) then the actual engine will be the same but the trigger board may be different. In which case, go for the V3.
The MP5K does (if memory serves) have a unique selector plate on it, so if your engine's fire selector mech is based around modifying/reading position of that then you might want to consider picking up a spare to mess with (It's possible this is only relevant to things like Perun Hybrids and not HPA, but worth mentioning).
