Fenix,
A few thoughts:
1. A 120 spring is still going to have you shooting near 380 fps with a .20 g bb. You'll probably want a 90 or so spring for indoor use;
2. If you want to use the gun outdoors then you can keep the 120 spring and keep it under 400. You won't then need the MOSFET (saving you some money), keep full auto as an option and reduce wear and tear on your gun's innards;
3. I don't know about gearboxes, etc so I don't have technical insights to share about your gun's inner workings;
4. If I understand your post, you're planning on a 455 mm inner barrel? You may find it be a bit too long to drive in the tight quarters you'll encounter in CQB 1 and CQB 2;
Good luck with your baby!
EDIT: Misread your budgeted amount.