I fell into the trap of upgrading on my M4 and MP5. Partly because they were slightly damaged albeit cosmetically. Once you upgrade one part, there's always going to be something else you need to get to make it fit or look right or even operate properly.
For the MP5, I was fortunate to have started the upgrade because the internals were crap. The M4 was because it was missing parts and was held together with duct tape when I got it. Since then I've broke the mechbox and hopup unit so it's a totally new gun.