off the top of my head if you are using the MP5 for both indoor and outdoor, you will want 350fps. no more. Also, you will want to source a good gundoc on your area to install these parts for you.
Here are some general upgrades that you might want to consider for the internals. externally, that is your business:
Piston: anything by prometheus, modify or guarder
Piston head: anything by prometheus, modify, guarder or ARS. (i recommend the latter)
Cylinder head: anything by prometheus, modify, guarder or ARS. (i recommend the latter)
Tightbore inner barrel: anything by Prometheus, KM, or Madbull (prometheus being generally considered to be the best of the list). I dont like the systema ones. id say stay away from those.
Air nozzle: anything by Prometheus, guarder or modify
Spring: PDI, Laylax/Prometheus etc are all good choices.
Again these are some general upgrades. These will provide accuracy, reliability and longevity. If you want ROF increase, you will need to change the gears as well as buy a new motor. Its up to you in the end. the upgrades i listed are basic ones that will do any gun a world of good and will improve performance tremendously.