Buying an incomplete project

This is my first trip into the homebuilt world. I'm probably going to
buy a 90% complete project, and want to know if there will be any
paperwork hassles I'll have to jump through to get it registered.

I realize I won't have the repair authorization as I did not build 51%
of the plane, but what should I look for as far as documentation from
the current owner and what will the FAA expect to see from me?

I have AC 20-27F, but during a very quick glance-through I didn't
notice any specific mentions. Any help appreciated!