If you have never worked with fixing a Mustang you are in for a HUGE shock. If you do a ground up rebuilt just the materials like sand paper, filler, primer, sealer and color and clear coats will run you right $3,000 to $3,500 just for materials. A ground up restore will require probably 2,000 hours of labor and that is $50 - $100 and hour. The TV shows doing a car in a week or 10 days is BS. There are shops that do specialize in just rust repair and panel replacement. Here is a link to one outside of Atlanta, Ga. .
If you just have lots of surface rust you can get the car dipped but that will cost over $2,000 and you have to remove everything and send just empty body shell.
You will come out ahead buying one already done and let them take the losses.
https://www.graverobberssheetmetal.com/index.php/price-lists/mustang-prices
Graverobbers does not body or paint work and after the repo panels are on you should remove all the black e-coat and put on good epoxy primer and go from there. No you are not going to get a perfect body back takes lots of custom panel work if you want one perfect and Graverobbers is not a restorations shop only panel replacement.
A really good restore will be right up around $100,000 unless you just make one a shiny used car like most do and not make it concourse.
I have 5 convertibles two are parts cars. You can find good rust free ones for decent price if you shop around. This yellow convertible I got off Ebay for $7,200 and is about 80% original paint and interior. California car no rust in floors, trunk or quarters. I just drive it. Has 351 C, c-4, tilt column, gauges, upgraded to 4-V is H code, Edelbrock performer intake, MSD, console, Decor group, convenience group, deluxe bumper, dual mirrors, sport wheel covers, AC, tinted glass, PDB, PS. 48,000 miles. Came with the original steel wheels and caps and a new set of aluminum wheels and Firestone tires. I just cleaned the car up and checked everything out. He had put one head gasket on backwards which I fixed.