I rebuilt my 156 a few months ago (Cat D) after a front end argument with a deer. MOT is void once the car is written off, Cat C means VIC check.
Don't be fooled by an MOT or VIC check. These will not ensure car is OK. Insurance companies are a bit wooly on cat's. Generally was told cat c means chassis damage, they wanted to cat c mine, I complained because on there was no chassis damage, they changed to cat D.
The thing is there are bargains out there, but they are very very hard to distinguish from dogs, especially if you don't know what you are looking for. Would suspect this has had a earlier bonnet dropped on, got vic and MOT, but either still has issues, or just a bit of the usual alfaitis. If you bought it you would never be happy with the bonnet, so assume you had to change that, sort out the other problems, and still have the stigma, and possible issues from the cat c. How much ? s/h bonnet plus £500 to paint, plus other problems....
If you are interested why not get all your questions together and phone the bloke selling it and ask him. If it doesn't sound 100% all the way, just walk.
Personally I wouldn't bother. But if you really did, you should be looking for photographic repair, or professional receipts, 4 wheel laser alignment, and evidence that the chassis etc is straight, and if anything else got bent in the shunt. Before you even bothered to look.
Be very careful and get someone to look at it who really knows what they are doing if interested.