What to look for
I'm building with old RV-6A kits that were delivered between 1992 and 2000 and were stored indoors. It's been basically fine. The types of corrosion to check for are at the exposed edges of aluminum sheet, especially if there is vinyl cladding, for filiform corrosion. Bare steel may rust, check especially the machined ends of the gear legs and the sockets in the engine mount because they must be close fit and removing rust will cause looseness and problems. Also check that vinyl cladding on aluminum sheeting can still peel off. Once it gets brittle, if exposed to UV, it is almost impossible to remove. Bare aluminum should not be corroded if it has been kept dry and clean and dust free.
Powder coated parts may have rust coming through because the early powder coating was poor quality. It can be stripped off back to bare metal and re-finished.
Hardware should be all OK. There may be some residue to remove from masking tape and label glue that has perished and hardened.
Check also your own abilities to construct an older style kit. There are errors in the plans and the instructions lack detail and content so you need to be resourceful and good at independent problem solving. The new kits are much better and the manuals are vastly improved, like assembling Ikea furniture. So the self check is as important as the kit, I don't know what is your experience, but you must be looking for a challenge.
If parts are missing or become damaged then there is dimensional info in the plans to enable you to fabricate your own parts from scratch.
Good luck, it's may be more fun than following a step by step recipe.
p.s. also check that there is original bill of sale so Vans can note you as the new owner to qualify for engine and prop special pricing and the new policy for parts supply.