It's very hard to improve the value of a car without spending more than you can get out of it. Best plan, if it is clean and in good repair you should get a decent price. You mention door panels, if the interior is in good shape except for a ripped door panel it will pay to fix just the bad part.
I don't know about the legal requirements in your area. But here, a car must pass a safety inspection before it can be registered for the road. Having this inspection certificate can add a lot to the desirability, and price while it does not cost much if the vehicle does not need any repairs. Therefore, if the car is good it pays to get the safety inspection.
Don't try to squeeze every penny out of it or you could be waiting forever for a sucker to buy it. Don't put too much into it, price it on the low side and it will sell. It is a big mistake to spend a lot of money then think "I have to get my money back" maybe you do but the buyer doesn't have to give it to you.
As for the price look around the ads in your area and see what similar units are going for.