well for me the piston head is the biggest priority part to switch out, but the piston itself is poor quality the gears even seem to wear on it pretty hard. so if your changing one I would say change out the whole thing, Piston/head. But I would also say before upgrading just game the gun for a bit. no need to upgrade right away. sure it may not be super consistent, but until you have a feel for the game, having a better gun wont make much difference. My Mp5 is still completely stock, and it dose the job well enough for smaller CQB fields.
All in all I would say if your looking for a good gun for a good price the G&G MP5SD has done well for me. But looking back I probably would have bought this instead
ICS mp5 ,I believe locally here it goes for $389 so not much more cost wise then the G&G
M4, L85, Mp5, Scar, Sig 552, M14