I’ve had through my hands (at last count) 10 MP9’s, most of which were broken, so can make decent list of stuff you will need at some point. I’ll go through your list first:
Part 153 (disconnector) - yes, very much needed. The main MP9 point of failure!
Part 151 (hammer release) - not seen a standard one break, but if you get a steel hammer, then get this too
WiiTech piston head - yes, but double check this doesn’t need to be used on the WiiTech piston chassis.
Part 25 - not seen a broken one, I wouldn’t bother
Nozzle springs - yes, get 2 sets!
My recommendations:
TNT Barrel set - so much better than the KWA hop unit
WiiTech Steel Hammer (part 26) - standard ones snap at the sides, sometimes will still function, but best to change to steel. Plus it’s a pain to change, so get this and only do it once!
WiiTech Steel Action Bar (trigger bar) - I have one on mine and makes the trigger pull much better
WiiTech 130% Recoil spring - MP9’s can have issues if they don’t fully go into battery, this will solve that
A CowCow/LPE/WiiTech piston head. Anything over the standard KWA piston head. They dry out and crumble. Beware there are 2 different sizes of pistons and piston heads, 14mm (orange) & 15mm (green), so work out which you have.
If you already have a WiiTech nozzle I’d get the WiiTech piston chassis. The WiiTech nozzle will destroy the back of the chassis eventually. Also the WiiTech nozzle is very difficult/almost impossible to change the valve on….
Hope this helps. Enjoy the MP9, it’s such a little beast ?