If you take the attitude that a Q is your destiny then no problem. The rally boys used to used them to avoid the emissions tests on the MoT for tuned engines, but that will go in the new MoT rules from Jan 2012 onwards.
The IVA test will make no difference to the registration, that comes later when you register it with the DVLA and the build quality inspection that will need to be done.
The Q reg is the catch-all-bucket that DVLA use when there is not legitimate place anywhere else they can put it.
For instance for my Suzuki bike all I needed was a letter from the Suzuki owners club verifying that the frame was the age I claimed (£25) job done. I have know a Cat B Nobel that was repaired and when the V5 was requested the DVLA sent the requester the V5C, result
In your case, and I would guess and for a number of builders its the evidence that will be difficult to get that will allow the DVLA to give you an age related if you want it, if you cannot get or find the evidence then they give you a Q to get you on the road.
In your case if the car was salvage or scrapped there would have been a CoD (Certificate of Destruction) from the authorised scrap year.
If you contact the appropriate specialist club or group (there is always one somewhere) they will be able to identify say, the engine number, and detail when it was originally used on the production line, a headed letter later and you have the age of your engine.
With a little effort and some pointers there is a chance you can avoid a Q, but you need to be diligent and committed to the investigation.
Personally I think a Q plate is unfortunate, it does not detract from the car or the build however, if I could avoid one by a little (or a lot
) of effort I would go a long way to achieve it myself.
Personal view you understand, you need to make your own mind up what you want.
-