Welcome (almost
) to the ancient art of building a non pre-punched RV kit.
What to look for on the partially built kit: Kind of depends on how much has been completed on the unfinished kit, and to some degree the kit vintage. Other than general build quality (fit and finish, properly set rivets, etc), there are some specific things to look at for any early RV model (3, 4, 6/6A), specifically where the major attach points have to be drilled by the builder. These include empennage attach bolts, wing rear spar attach bolt (on the 6/6A), landing gear mount alignment (6A: early ones all done by the builder....later ones had gear leg to gear mount hole done at the factory).
Replacement parts from Van's: Van's has said they will continue to support all their kits. That said, there are sometimes wait times for some parts that are not in volume production (like the 3, 4, 6/6A kits). If you need to order a sub-kit for something not included in the one you're looking at, read this on Van's website:
www.vansaircraft.com
Making 7/7A parts fit a 6A: Depends on what part we're talking about. Generally if the newer model uses the same part number as the 6/6A, the part will fit. (my 6A has some 7/7A center console bits in it). If you're talking about swapping wings and fuselages.....no, those are not interchangeable. 7/7A empennage assemblies.....those can be "made to fit" on a 6A.
There are several of us around here who've been down the "old school" RV build process.. If/when you have more specific questions, we can help. Semi-pro tip: photos will help get better answers to the "Is this OK?" type of question.