[Big Pepa]

If you want a 120 spring (in god's name I don't know why considering a M90 spring hurts plenty) then all you really need is metal bushings and a good bearing spring guide. Sorbo pad is optional but doesn't hurt.

M1000 motor is a waste of money as the stock motor is good. If you are looking for higher RoF then there are better motors out there.

Buying a new cylinder is a complete waste of cash unless you are planing to fit a longer (or shorter) barrel.

I'd take the money from the cylinder and the motor and invest it in a tightbore barrel myself... but that's personal preference.

Mosfet's rule... In my opinion one of the single best upgrades for a gun. You won't be disappointed. Rewiring is a big pain in the ass though. Get one with a 3 bb burst function... they are fun.

Since you are planning on a 120 spring an O-ring air nozzle is a waste of money. The 120 is going to crank out plenty of power. The O-ring will make a marginal difference.

I'd buy new gears, but, you don't have too. A Prometheus torque up set would work nice (so would a high speed set). The only reason I'd go with new gears is that they will last way way way longer then stock. Really though... they are optional.

To sum up...
required...
M120 spring
metal bushings
spring guide

Good investment
Sorbo pad
Mosfet

Optional
Anything else