The reason explained to me when I was issued a Q plate ( I scratch built a couple of trikes a few years back) was to denote the vehicle was of uncertain or unknown manufacture or origin, and was to protect future buyers as, once issued, a Q plate can never be registered with a different number ( private plate etc.) This was in spite of having all the invoices for materials and new parts used in the builds which, in theory, should have allowed me a 'new' registration.
In practice, unless the vehicle was made and sold into the UK market as new from a recognised source, the DVLA's preference is to issue a Q plate whenever they can, and as its such a long and laborious process to argue the point to get a proper reg. number, most people dont ( me included)
Theres lots of valid reasons that a vehicle can carry a Q plate (imports, mods, rebuilds) and you have to remember that reg. numbers, Q or otherwise, aren't handed out willy nilly to any old vehicle. A lot of Q plate vehicle have undergone a fairly strict VOSA examination in order to get a validation cert. for registration with DVLA.
Its probably worth, as previously advised, to try a few insurance quotes with the Q plate. But I can honestly say that not only did i have no trouble insuring my Q plates, but I didn't have to pay over the odds either. And I certainly had no trouble selling them.
If your happy with the vehicle you've found and it has all the paperwork relating to the issue of the plate then, subject to satisfactory insurance, I wouldn't worry about it.
Best of luck
Paul
