is that it, is that all thats wrong with it?
if so you damn right its worth putting back on road.
as above, wings ar cheap, so are the filler repair panels, but a pattern part filler repair panel will be crap, so try and find a good fiesta in scrappy and cut the back part off that.
wings are easy to change, if you have a welder, but you can hire them.
spraying is different, i would recommend a body shop for that unless you know how to do it and have a place for it.
the sill is nothing, there cheap as chips to buy, but if ya want, cut out old rusty bit and weld in a bit of metal, good enough.
you got to ask your self what you feel better doing, if patching it is ok with you do that, if its a new seal (which is harder work) do that.
find out what bits need doing, call a few body shops, tell them your thinking of buying this car but how much roughly will you be looking at for them to do work.
find a back street place and call them there be cheaper, but remember, in most cases you get what you pay for, this inc cars, if you buy it for penuts dont expect a half minter.
as for price on this car, if it has not T+T i would pay more than £300 unless the guy took it to aMOT garge for a mot, even if it failed i would want a fail sheet, but not more than £500 tbh
however if it only failed a MOT on the sill say, then it could be worth more to you, but price up repairs, make sure you have a fail sheet for mot (if it hasnt mot) and price in to put that right, look at what money you got to spend and go from there.
only you can truly decide what its worth but dont pay even low book price for it if it hasnt a MOT.
defo worth putting back on road IMO, but then i can do all work my self so would be cheap for me.
if you do get it to break it let me know mate.