Refer to FAA Order 8900.1 Volume 5, Chapter 5, Section 5. It states in part:
5-1219 ELIGIBILITY REQUIREMENTS.
A. Residency and Age. The applicant must be a U.S. citizen or an individual admitted for permanent residence in the United States. The applicant must also be at least 18 years of age, and the primary builder of the aircraft.
1) When a club, school, or partnership builds an aircraft, the FAA considers only one individual (such as the class instructor or designated project leader) for issuance of a Repairman Certificate for that aircraft.
2) An individual working alone who applies for a Repairman Certificate must build the majority of the aircraft in order to be eligible for the certificate.
The above is what the ASI will be using when he makes his decision. So if you are an individual who takes over a partially built kit that is more than 50% complete then you might be denied the repairman certificate. And the higher the level of completion when you take over, the less likely you will get the certificate. In other words, don't expect that by simply installing the prop on an otherwise completed project that you will be qualified for the repairman certificate.
Still, as long as the plane meets the 51% rule of amateur built, you will get the airworthiness certificate and be able to do all of your own maintenance but will just need an A&P for the annual condition inspection which he will most likely accept your owner assistance on. So your maintenance costs will still be very reasonable. Nothing to fret about.