Despite wanting to recoup as much of the cost of getting a car into a sound condition as possible, you can't really count expect to get anything like what you spend on a BX.
I have to say, given the cosmetic condition of the car, guaranteed age-related issues and the fact it's an early Mk. 2 16 petrol, you will struggle I'm afraid to sell the car. OK it's a rare special (but only of interest to some BX buffs) and has new MOT.
Sad to say, but (this is only my opinion) your best bet is eBay, but you should be pragmatic and expect a couple of hundred.
However, a day spent cleaning, T-cutting polishing will help - and tidy the wheels (try some cheapo wheeltrims too) - but that dented door will put many off spending too much.
So what to do? Tidy her up and use her for a few more months - then sell with 6+ months MOT and a couple of month's tax...?
Wishing you good luck Carolyn.