Charles,
A DAR is a Designated Airworthiness Representative. That is a person that has been certified by the FAA to do inspections to determine eligibility for, and issuance of, an airworthiness certificate.
This person may or may not be a good person to have complete a detailed condition/pre-buy inspection.
A better choice would be someone that has extensive experience with building the RV model you are considering, and even better would be if they are also an A&P, but not essential.
Deer Valley Airport in the Phoenix area has considerable RV activity. You might make some calls and round up contacts there for RVers that might be able to assist with the pre-buy.
Also try the VAF White Pages:
http://www.vansairforce.net/rvwp.htm