These are stickied in the Upgrades & Mods section
Smart and Not So Smart Upgrades
Why do I need to Upgrade
As an aside...sorry that you picked a Cyma/TSD MP5...they're so wonky and crap that you'll be more fighting the loose tolerances (mechbox, parts and body) than learning anything.
It's sounds odd...but get a good mechbox to "practice" on...or at least start with a good shell if you're building it from scratch. The parts stay in place, more parts will fit properly, they'll tend to stay in place while under tension a bit easier.....
With bodies...you can't go too far wrong with an M4 for a common platform. But AK/UMP/G36 AEGs are actually easier to work on (they're all version 3 mechboxes...which is 90% the same as a V2). There's less to take apart to get the mechbox in and out...so you can try something and then test it out without having to reassemble the whole thing.
Anyways...best of luck. Degrease (just wipe the parts off with a towel) and work on shimming first. Plastic bushings or not...the concepts the same.
Tys