Does anyone know if I could buy the Guitar Rig 2 controller and use the software for Guitar Rig 3 (so it would essentially GR3 with 2 less buttons), because I dont want to spend over 100 quid more just for 2 buttons.

Yes, that is the configuration I use.

I use a PC laptop, not a Mac, so not sure about the Mac.
BC Rich Mockingbird Supreme
BC Rich Wave
BC Rich Virgin 7-string (w EMG 707s)
Parker Fly Deluxe Hardtail
Gibson Sonex (Blackouts Metal and EMG 89)

Guitar Rig
Ableton Live