I got my sexy ICS Full Metal from Styrak, and love it :P
Only problem is a few things are proprietary *can be pretty expensive for some stuff*, here are some common ones:
- Stock (Modification can be made to fit others)
- Anti-reversal latch
- Pistol Grip (Modification can be made to fit others)
- Pinion Gear
- Gearbox Shell (Most people who like ICS like it because of a much easier gearbox to work with such as changing springs easily)
Other stuff recommended to upgrade:
- Piston + Piston Head
- Inner Barrel
These are easy fixes I guess...
You can always refer to the ICS Bible for more info...
*Takes like under 2 minutes to fully disassemble it too, although you could probably do the same with other ones
*
Reviews on the SRC M4 Gen III Pro also seems to be pretty good, only set back is fake trades (some people care). You probably just have to downgrade the spring for your field limits when you get it...