so if someone sells a 75% completed plane, and i finish it, i could have the original builder apply for the repairmans certificate (if he was local and convenient), or if i thought i knew enough of the plane i could apply for it myself?
Let me start by saying I'm not an expert... I've never even stayed at a Holiday Inn.
My understanding is that even if you buy an already flying experimental airplane then you, as the owner, can do maintenance, repairs, improvements, modifications and basically everything except sign off on annual condition inspections. If you bought a flying RV you would need an A&P (or the one person specifically authorized by the FAA do sign offs on that airplane) to sign off on the annual condition inspections for you each year.
So, if you try to become authorized to sign off on annual condition inspections and they turn you down, you are still able to do much more of your own work than, say, the owner of a 172.
If I were in your situation, I would try to convince the FAA that since I finished the airplane and determined that it was ready for inspection by a DAR, that I was essentially signing off on the 'first' condition inspection. If the DAR agrees that it is ready for phase 1 then arguably, I had a good understanding of the airplane's condition. Since they didn't have a problem with me doing that for the first time, then they shouldn't have a problem with me doing basically the same thing again each year.
At least that's what I would tell them, they of course get to make the decision.
Even if you don't get authorization from the FAA to sign off on annual conditional inspections, you still won't be in a bad situation. As long as you don't misrepresent anything, the worst that will happen is that the FAA tells you "no" you need to have a professional airplane mechanic sign off on the airplane's condition once a year. Your maintenance costs, if you work on the plane yourself, should still be much lower than owning a certified airplane.
Maybe Mel, or someone else who actually knows what they're talking about will chime in.
I wish you the best with your decision. Actually, if I were buying a partially built kit I'd be more worried about the quality of the workmanship than signing off on annual condition inspections. I'm critical enough of my own craftsmanship -- I'd never get to sleep at night if I had to think about mistakes somebody else may have made on my airplane that I didn't even know about.