Work out the difference. If you can spend £500 but make £1000 more because of it, then you're £500 up so it's a good idea. If you're going to come off even then I'd still do it anyway as it'll make it easier to sell, but if you're going to come out down then it's not worth it.
Personally, if I rock up to view a car and the seller hasn't disclosed just how bad the bodywork is, then I'm going to be extremely annoyed at wasting my time and will definitely be walking away. On the other hand, if you tell me exactly how bad it is then unless it's up for a bargain price I'm not coming to view in the first place. At this point, and given 350Z values, you're either going to have to let it go for about £1500 under list OR get the work done.