IMO, CA has the better externals, but KA has the better internals. If you want a gun that you can just literally pick up and never screw with for a long time, the KA is your best bet. If you're looking to build off an upgrade platform, I'd recommend the CA.
Mags are typically interchangeable, but there's always a few brands that don't like each other for some reason.
IOTVs are way too big. The 88 is fine, but you may want to get something that you can modify for your needs (read: MOLLE compatible). There are shit tonnes of different kinds of vests out there.
I change primaries like other people change socks.