You don't need a DAR to do this. This is straight forward (for Government). With a little research you can navigate the paper trail. First place is FAA web site.
Must Read
https://www.faa.gov/licenses_certificates/aircraft_certification/aircraft_registry/reregistration/
https://www.faa.gov/licenses_certif...ication/aircraft_registry/reregistration_faq/
https://www.faa.gov/aircraft/air_cert/airworthiness_certification/std_awcert/
https://www.faa.gov/documentlibrary/media/form/ac8050-2.pdf
You are making a NEW registration. You will go through the registration process from the start with a new N number. You will need to get the sales paper work. If you want a special N-number now is the time to reserve and request it. Otherwise they will assign something. The old number may NOT be available. I believe it is locked for 5 years after it expired. CALL FAA.
As far as airworthiness the plane should still have it's airworthiness since this does not expire, unless it was surrendered or reported destroyed. You can check this and it should be onboard the plane. CALL FAA...
The plane is not a pile of parts due to expired registration. However it is grounded until you take care of it. It could be a pile of parts if it is not safe, airworthy and comply with all the FAR's regarding airworthiness, like current condition inspection, current pitot static, all required equipment (like ADS-B if you fly through airspace requiring it), operating manual.
Do you belong to the EAA? If not join and go to local chapter meeting. Find the local Tech Advisors and RV pilots. You should have them look it over before it is flown. If you have no RV experiance get some dual in another RV. Consider another pilot doing first flight after coming out of mothballs. I assume it has not flown since it is illegal to fly with expired registration.
If this has been collecting dust for a long time, you might want to treat this as a NEW experimental aircraft. First obviously a complete air frame and engine inspection and services.It will need a condition inspetion by an A&P. Recommend you redo W&B and flight manual (which is required to have). The flight manual is developed during Phase 1 flight test. You may want to redo part of the Phase 1 flight tests at least in part.
FAA Oklahoma number for registration will get someone that does AC registrations all day. Because you are buying an un-registered plane, you will need to coordinate that with FAA. I don't think you can buy it (at least in FAA's books) until it is registered. Not sure if owner needs to register it and you then buy it. How you exchange money is up to you. Make sure it is not under a lien and they actually are the owner.