Personally I would start off with a bid of £500 explaining that the main dealers charge £950 (thats what Seaward MGRover want to charge me) for just the cambelt change and you would have to get that done first as you don't know the history.
Of course if you do or do not get the belts done is up to you... just tell the seller that
Oh and you could ask what the reserve is, give you an idea what he is expecting?
Also.. .if there are "very minor dinks in the roof" why are there no pictures of the roof to show how "Minor" they are?