The rules of the DVLA are clear once you get to the words you need to read.
In a 'nut shell', you need the invoice of the kit to prove it’s new with all the new parts listed for clarity or a sales sheet detailing the contents of the std kit of bits. If you buy a part completed kit it is essential you get that invoice or you are ‘Q plated’ or in the case of Robin Hood you have to pay for a chassis plate and a statement of age declaration (been there, done that).
They judge the car on what they refer to as the 5 major components of the car to determine its age classification: - Engine, suspension, transmission, body and chassis.
You are allowed one refurbished item in that group and they will want to see the invoices for each claimed new and refurbished item.
In most cases the engine as well as the transmission i.e. gearbox- rear diff drive shafts etc. could well be donor provided so you need to make sure you are covered, they do not dwell on the smaller components like the drive shafts but if there is a debate, invoices to prove they are new could swing the argument. At least clean and paint them so they look the part.
The VIC inspector if he attends your workshop does not make the decision, he collects the data and evidence and the judgement is made back at the office with his recommendation taken into account.
I have heard that if the Speedo reading is greater then 10 miles that is a negative so watch the test run mileage as you need all the positives you can get as there is little chance of appeal.
As I am on their mailing list as a builder I now get their regular post based news letter, I am sure that they would add others on request. One point they stated recently was that the DVLA now expect photographic evidence of donor’d parts, the old V5 is not sufficient to acquire an age related plate so keep a full record of the change process from rusty heap to butterfly, taking special care to capture the serial numbers where they exist, on the way through.
Hope this helps