Seeing as the Scar has a standard V2 gearbox, I would go for the Perun Hybrid V2 rather than the AB++ this will give you a lot more features and reliability especially if you are running 12:1 gears (personally I would just stick to 13:1 and you can definitely find them cheaper than that anyway -check AK2M4.co.uk he is cheaper for the ZCI barrel you have listed as well). Your motor is a similar story, I would go with an ZCI, SHS or XT brand one for about £30, the one you have isn't that special and is overpriced IMO. If you want better performance and don't mind spending a bit more, go with a warhead brushless. Everything else looks fine, not a massive fan of SHS ali nozzles as I had my M14 one snap instantly when dry firing the gearbox without even having a hop unit on (looking at it afterwards it was so thin I can't believe how poor the design was and I had to switch back to stock).
I have several double bell P90s and honestly couldn't be more impressed with their stock internals, the stock pistons are some of the best pistons out there and their gears and shimming all seem fine from the factory. It is amazing how good they can be out of the box with only un upgraded hop rubber and a brushless motor without even touching the gearbox. I have been eyeing up the Scar for a while but haven't managed to justify pulling the trigger just yet.
just realised after writing that that you have a replacement piston in your list. If it has the black piston with full metal rack and the red aluminium piston head already in your Scar I would just use that. I would buy up these pistons if I could find a place to as I love them and haven't had any problems with them in anything. If you still want to change your piston then you will probably need a new piston head as the piston heads tend to be thread locked on so tightly that it is impossible to separate them from the piston themselves.