Yes a big call here. You need to break it down cost wise and make a decision.
Wings are not an issue, a pair of new wings cost in the region of £300.00, plus fitting and painting. If it goes into a body shop you can add £600.00 pound plus for this including the boot lid. Or, you can purchase second hand wings, which cost peanuts on eBay, but you will need to be really selective, as these may well be pretty rusty but serviceable.
The bigger issue is the sills, if the sills are rotten, then it probably would not pass its mot anyway as this is an integral part of the safety structure. So the decision on scrapping the car would be made for you.
The priority here is the cost of the sills, and the removal and welding in place, you also mention the wheel arches, I take it the rear arches here, this rot would need cutting out and repairing, so it looks as if a total respray is just about on the cards, so an expensive total repair, probably costing in EXCESS of £1500.00.
That is just a guess and I am sure other people will have a better idea than me.
So, an mot failure is on the cards, when is this due by the way? Followed by a sorn and restoration project, or just scrap it!
Unfortunately these cars due to their age do take a fat wallet to maintain, and unless you are very handy and a half decent mechanic, with mates in a body shop they will cause a bit of hardship.
What else looks in need of repair? Exhaust, underbody coolant pipes, how many miles has it done?
Good luck here, it is always a shame when the tin worm takes hold, but sometimes the cost of maintenance can be very prohibitive.
Guys, any other comments?