51%
My understanding is that the 51% rule has nothing to do with actually performing 51% of all of the work, i.e. driving 51% of the rivets. The FAA actually looks at the build by tasks. For example if you debur, flute, drill and rivet a wing rib, it doesn't matter if you do them all or just the 1, you are given credit for that part of the 51% of the tasks. This is how the QBs qualify for the 51% rule. Putting on the 1 finishing skins is considered the same as putting on all of the skins.
I am probably being too general in this explanation and others can comment in more detail. I don't know that there is a list of the tasks that the FAA considers in determining majority build but there must be one some where.
With that said, I don't see a problem in continuing with a kit that is at the stage you are talking about. There is plenty to do that should meet all of the requirments. Start a log as if you were starting from scratch and document your efforts accordingly and there shouldn't be an issue. Of course to be sure it may be a good idea to contact your nearest FSDO and ask them.
If the work done is of good quality, you are sure you will get all of the parts, the kit is of relatively recent vintage, and the price is right, I would sure go with it.
But on the other hand there is something satisfying about ordering and receiving a brand new kit and building everything from start to finish. Then you can truly say it is all yours rather than saying I built everything except the empenage! I think this is the reason I will probably go with a new slow build kit rather than buying a flying RV or even a quickbuild. There is just something that appeals to me about building the whole thing rather than flying a nice plane that someone else put together. I guess this could be taken to the extreme and by saying the only true builders are those who design there own plans and build from scratch, but I am not up to that task and draw the line between the slowbuild and quickbuilds. To each his/her own.
On the other - other hand, I am not one to pass up a good deal. I almost purchased a nearly completed RV7 but didn't pull the trigger quickly enough and lost the deal to another person more decisive then I. Value talks so you can't go wrong. If is right for you then go for it. I don't see any real technical risks.