I agree on the Systema comments above. They're not exactly renowned for their non-PTW stuff. I'd say something much nastier, but I finally met wildcard in person and he can now identify me on the street... so.. you know.
Your budget is actually
almost too much but you can do some very nice things with it...
If I had that budget for your build, I'd probably get:
- Lonex V3 gearbox + sorbothane
- High torque neo motor (probably a lonex but anything with STRONG magnets is fine by me)
- MOSFET, Gate GDS-4005 or Extreme-Fire or something simple home-made
- rewire for Deans or XT60 plus fatter wires
- either install R-Hop, Modify's flat nub+bucking, or a PDI W-Hold
Leave the stock hopup unit, leave the stock barrel (maybe give it a good clean), shim the hell out of the gearbox, perhaps lighten the piston, and call it a day.
If you do get the Lonex V3 gearbox, make sure to AoE the piston with care if you're planning to feed this gun a lot of juice and torque.