The Amateur Built classification requires that the total project meets the 51% rule. That is, the 'manufacturer' - that's you AND the guys who started before you - has to provide 51% or more of the labor involved.
You shouldn't have any trouble at all with the Amateur Built portion of the requirement. You could buy a 100% completed homebuilt, fly off the test hours, and it's still a homebuilt. You just didn't build it.
To get the Repairman's Certificate on that aircraft, *you* have to be the primary builder. Even though there's a lot already done, the remaining work may qualify. To finish the project, you'll probably have to be very familiar with all its various systems. The DAR may allow you to get the Repairman's priveleges on the final product if you can convince him/her that you are, indeed, the primary builder.
Even if you can't the the certificate, you can, as Dan stated, have an A&P (not necessarily IA) do the annual Conditional Inspections. You can do all the repairs and maintenance and log the work. Each year, it needs to be examined by a licensed mechanic or the guy who has the Repairman's Certificate on *that* aircraft.
Disclaimer: I reserve the right to be wrong!
Jon B.