Hi Dean
To give yourself the best chance of an age related number plate, ideally, you would use a single donor.
The impotant thing is to keep your paperwork in tip top order .... Donor cars V5 on SORN & registered to you is a good starting point
That's not to say that you can't swap some parts for new ones or like for like.
In my own case, all of the parts from a single donor exept the engine .... In theory I should still qualify for an age related plate ( Issued under the points system )
That said, I know that some local DVLA offices can be difficult to deal with & have different views as to what is allowed ( In spite of the information contained in their own web site !!?? )
I'll request an age related plate & if I am given a Q, i'll not loose any sleep over it