Matrix parts are shit. If you're going to get an FTK (Full Tune-up Kit) Guarder, Modify, and HurricanE get my votes. If you want parts a la carte then SHS and Lonex will never fail you. The user "Stealth" runs
Airsoftstore.ca. Good guy from what I hear and quite well respected, also has a bunch of SHS, Lonex, and is now carrying "Core" stuff from what I understand.
Also don't upgrade just for the sake of upgrading. Preventative maintenance is one thing but upgrading for the sake of upgrading is another. Take mine (and other senior members) words of advice; "Upgrade the player not the gun". Work on your own skills first and don't use an upgraded gun as a crutch. Cause I've been shot with a $15 Canadian tire special before by a sneaky as fuck guy.
However, first two upgrades I'd recommend after you've had a bit of time to play around with the gun in stock form are Hop up bucking and inner barrel.