Cat D is neither here nor there, it is reserved for cars that have no structural damage but are beyond economic repair. I can render my old car Cat D by keying the side. Or I could if anyone would notice one extra scratch. I don't think that you need a VIC to put it back on the road, after all it has had no structural damage.
Cat C *does* need a VIC, but once obtained you can re-register the vehicle and you are in business, you don't need another. You do have to declare a Cat C as such every time you sell it, which will depress the value and make it harder to sell. I'd therefore suggest that you factor this in when buying. If you plan to keep it for a long time then this isn't a problem as it will be worthless by then in any case.
The only other caution I would advise is to check the quality of the repair. Many years ago I had a Fiesta that I damaged, the insurers had a new front end put on and they didn't bother with any real rustproofing. I had to spend the next 3 or 4 years patching up the paintwork on the front, when I tried to claim on the paint warranty they refused it because it had been repaired outside the Ford network. Many insurance repairs are not completed to the same standard as the original maufacture.