I am quite fond of our 1980 Invader, but since the 81 has the softer ski springs and the vari-ride suspension I am looking forward to completing the re-build of the 81.
Be sure to check out the "So you got a "New to you Kawasaki snowmobile"" thread. There is quite a bit of good information there. Others can share their thoughts. After the first few learning projects we now replace pistons, rings, crank seals, carb jets, filters, fuel lines, and fluids before trying to run or ride a new-to-us sled.