If the battery runs down as you drive it is probably either a bad alternator or bad battery. You should send a note to Jeff Classic on the forum here or PM me and I will give you his number as he is a good fixer and a swell guy.
A working car is hard to sell just now and a non-working one will be close to impossible. I am not familiar with your car , but I can tell you that any Cobra that looks ok and runs well is worth all of your estimate. Get it running this week if you can and take it to either the Good Guys at Westworld on the weekend or take it to the Pavillions with a for sale pamphlet on the seat. You can test the water pretty cheaply..
