About proprietary parts, the gearbox shells and the receiver(upper and lower) are differents from standard market pieces. Then the first gear too if I remember correctely.
The rest is 99.99% compatible. You can check on the ICS M4 thread to be sure.
About the externals, you will have no problem to adapt sights, scopes, stocks (with little modifications though for some). The RAS are a bit pain in the arf to install if not ICS, you will, most of the time, have to adapt it to your receiver. And about finding a new metal receiver... it is impossible to get one as spare part.
If those two things aren't bothering you, then think about it.
And about dirt and water, I never had anything coming inside the shells from the exterior.
But hey I don't want to mess you with this
KA M4 are good too, but I love the split ICS mechbox. Really.
I had some issues with hop up once and in the middle of the field, I fixed it in 5 minutes.
And you can have two upper mechbox, one for cqb (less than 350fps) and one for exterior (more than 350fps). Changed in 30 sec. ^__^
Sorry again to mess up your ideas !
Go in the ICS M4 thread and see if it could fit yourself
Cheers,
f.