Well...get out your pad of paper and walk around the car, crawl under the car, fine tooth comb the car....and write down everything it needs.
Then pull out your catalogs and find out how much parts will cost you. Then figure out what parts you will do and what parts you will need to farm out to pros and get an estimate from them.
Some basics to get you started:
Decent paintjob 3-7k Varables are of course quality, body prep needed, and the amount of work you help contribute
Full motor rebuild 1-3k Variables are of course quality, full machine shop job, or you do assembly
You
can invest 7k into a mustang and have a real nice looking and running car when its done. But it will require very frugal sourcing of parts, using used when possible and some good solid relationships with shops/vendors to get stuff at good prices. You will need to do most of the work yourself and because of that you need to include in your budget 1-2k for the speciality tools and practice you will need in using them.
Here is a link to my resto spreadsheet:
http://www.dodgestang.com/death trap.xls
I have actually spent alot more than this because I had to build the motor twice and I am now on my 3rd tranny, but I tried to include just the items and costs involved with the car that would have it in the condition as it sits now if everything went right the first time.
You will see I have almost 20k into just restoring the car (not including the purchase price) and I haven't even done body work. I bought a repro hood, bolted it to the car and color matched the paint. I didn't fix the dent in the corner or any of the wrench fender rash that came with the car. I didn't need to fix any rust on the car. I basically did the entire
suspension with upgrades, the entrie driveline with upgrades, and the interior. Granted it is a more performance oriented build than you are looking to do
but it will help you know how much it really costs doing most of your own work.