Ken, welcome to M&M. I'm going to really start an exciting thread here as everyone chimes in their two cents. Hopefully i won't get chastised for any of this.
Here's mine...
What kind of answer are you looking for? Is the car worth buying? That can only be answered by you with what you intend to do with it.
I have been recreationally reading parts catalogs for about 15 years now. My wife always asks why. Last weekend we went to look at a car, and (among other things) the door handles were all scratched up, and she asks me how much they would cost to replace. Without missing a beat I said "$24.95 a pair from NPD." You gotta do your homework and be able to calculate what its going to cost you and make your decision from there. I spent most of my life too poor to actually buy anything and sat dreaming everytime I opened up the catalog and looked at the shiny new stuff I could buy for my decrepit mustang in the driveway and how much it would cost me whenever I did get the money.
My dad has been working on a 65 fastback for about 30 years now. Stuff like having kids, then building cars for them, etc, keeps getting in the way. Last time he had it to the body shop, he paid about $6000 for sheetmetal work and not a drop of paint was put on the car. First class work, the guy works the metal to the point that no filler is needed. To him its worth it because he has no intention of selling the car so resale value means nothing to him.
That's kind of excessive, but the point is you have to decide if you want to keep it, or keep resale value in mind as you restore the car.
Get a catalog and check out the prices for parts. Unless the car is first class original, and even then, everybody is going to spend about the same to fix up a mustang. It will cost you the relatively the same to redo the interior on a fastback or coupe, v8 or 6 cyl, 65 or 68. Cost you the same to rebuild the suspension or rebuild the engine too.
Here's the big stuff...
$600-1000 on the interior, depending on options. This gets you all new upholstery, carpet, dashpad, little stuff like sill plates and screw kits, etc. Pick up your missing seats at a swap meet for $50-100.
Brakes will run you about $400-500 to completely rebuild the drum brakes with new lines, cylinders, drums, springs, etc. Add about $500 to that (1000 total) to add disc brakes up front. Suicidal? brake shoes only from Autozone for about 30 bucks.
Suspension and steering can be rebuilt for another $700-800, and this includes all new control arms, springs, shocks, rubber bushings, rebuilt steering box, etc.
Nice wheels and tires can be had for $1000 a set.
That's all parts alone that just about anybody can do. Now for the fun part. Do you want to do the engine/transmission/rear end yourself or pay somebody?
Alex has a formula for a $1500 engine, but it entails doing the assembly yourself. Want a crate engine? $3500 and up easy from Ford Racing. Just want something that runs? $700 from Autozone for a long block.
Transmission parts run about $200 if you do the work yourself, multiply that by 3-4 to have somebody do it. A core can be had relatively cheap or even free for a C-4 or 3spd, you'll pay more for a 4spd or 5spd.
Rear end kit is about $80 for your 8", multiply that by 2-3 to have somebody do it. Add about $200 for a new gear set to change the ratio.
The biggest place you will lose money on a mustang is in bodywork. If you just want something shiny, go to Earl Shibe but he won't even wash the mud off before he shoots the paint for 100 bucks. Better paint jobs come from preparation, and you can either do the prep work yourself or pay a shop $80 an hour to do it for you. I've seen fantastic paint jobs for $3000-5000, and ludicrously nice for $10,000, but it always seem to be on a curve. Really, how much nicer of a paint job is something you spend $10,000 on over something you spend $5,000 on. Is it really twice as nice? I think not, but I digress. What about the parts you're missing. Fenders run about $130 a side, hood's about $150 and same for trunk lids. front and rear splash pans about $35 each. Headlight buckets are about $120 a side.
What else, wiring? You can completely re-wire most 65-67 mustangs for about $800 with brand new stuff. If you want you can sit and read a wiring diagram, you can fix anything that's cut with solderless connection kit and crimpers for $20.
Chrome? Bumpers, rocker moldings, all the "mustang" letters and 289 badges can be had new for about $400.
All the prices I've listed are ballpark for new reproduction, and I'm sure I missed LOTS of stuff, but they're fairly close for the average car.
From the sounds of it, you're looking at a parted out car needing almost everything. You can get stuff used either at a swap meet or on ebay, and you can expect to save about 50%, often more, on used stuff over the same part new. Over to your budget on what you can get new.