1. Check with the seller if the Madbull barrel is made to work with a stock APS2 hop up chamber, if not it is cut like an AEG barrel and will require an upgrade hop up chamber.
2. I'm not sure what field FPS restrictions are like around where you live, but if I got that Angel Custom kit I wouldn't use the spring it comes with, instead I'd just use the stock spring. Higher FPS setups will eventually destroy your gun unless you basically rebuilt the whole thing around the new spring.
3. The metal trigger assembly looks like a good idea if your gun was built with a plastic trigger box.
Edit: a cheap thing you can throw into this purchase which can help quite a bit is a sorbothane ("Sorbo") pad which you can glue to the end of the piston head. These will reduce the wear on your piston and make your gun quieter (sneaky!).
Airsoft is where expensive things go to die.
Last edited by Bellerophon; January 10th, 2013 at 20:27..