kyosho mp 777 wc interchageable parts

Basics:
(this is from memory)
SP2 and WC are almost completely interchangable (aka WC parts will fit a SP2 SP2 parts will fit a WC, but some of the WC parts might be "better")

for the drivetrain:
-The Universal swing shafts from a SP2 or WC can go in any buggy (the older buggies didn't have one piece parts, but could go into a SP2 or WC).
-The SP2/WC has a longer chassis then previous versions. The center CVDs are the same length front/rear on the SP2/WC, for the previous versions they were not (make sure the buy these as either SP2/WC or SP1/777/7.5 depending on what you have).
-Diffs are interchangable
-Mud gaurds are interchangable
-SP2/WC have more adjustable shock towers (more holes, for more setup options) but you could run an older tower on a newer buggy, or vice versa (I think).
-Suspension and shocks should be interchangable (although WC has the big bore shocks, which are better).

To be honest I am not sure what the differences between all of the models, I did just do a quick search and found that the 7.5RTR wasn't very good (cheaper parts, sorta like how the RTRs from Losi use much lower quality parts then the kits).
My suggestion is if you want to go kyosho (expensive parts that are hard to find) stick with the SP2 or WC, personally I run the SP2 (with random modifications, but not to WC spec) and I don't have any problems with it, other then part availablity.