Depends on what you're trying to accomplish.
FPS / ROF / accuracy / reliability
Can't have 'em all at the same time.
Also depends on where you intend on using it the most....outdoor field games vs. indoor CQB
Also depends on your playing style...hang back and plink at guys from a distance or kick down the door and be the first one through.
Quite frankly...if it's a new stock TM MP5 and you haven't played much...I'd leave it as is and not do anything. Play a bunch of games with it...both indoor and outdoor. You'll figure it out pretty quick what you'd like to improve first....after that it's pretty easy to figure out what parts/work is needed.
Best of luck,