Very first step is to find ALL of the rust. I would take a ball pein hammer and lots of light and go under the car. When you crawl out from under it be absolutely confident that the frame rails are solid and straight and know whether the floor has to be replaced and if the inner fenders and wheel houses need replacing or fixing.
When you have that established you can price out the parts needed, consult your friend to see if he really is going to help you until the end. Then you have to decide what you want for a finished product.
It's not necessary for every car to be painted and detailed underneath but if that's what you want the approach is much different than if you want a driver that you will be out in the rain with.
So anyway:
1) Find all rust.
2) Price and obtain necessary sheetmetal.
3) Either disassemble complete car or only whats
necessary (depending on results wanted).
4) Repair rust.
5) Finish bodywork and paint car.
6) Repair/refinish trim.
7) refinish interior.

When you do a budget quote, double or triple it and you'll be close.

Sheldon