In the '65 and '66 model years, fastbacks represent ten percent of the production, one in ten. They were a more expensive model and most were bought by performance minded people who wanted to share the "Shelby" magic without having to pay the price for a Shelby. Fastbacks almost always were ordered with v8 engines and hence were desired as used cars. Many were high school cars and most no longer exist. Hence, fastbacks are getting quite rare compared to Mustang coupes. The ratio today is probably one in fifty. That does not mean you can't find one. It does mean you will pay a higher price for everything associated with the car that is not common with the coupe. Fortunately, there are a lot of common parts between the car models. Unibody sheet metal is not always.
There are two approaches to owning a restored classic car. You can buy a restored car, or you can restore one. Of these choices, buying a restored car is cheaper in most cases than restoring one. The main reason this is true, is the cost of parts and labor. Labor costs are now about $60 per hour for any kind of auto work. Thus, if a car needs two hundred hours work (and this is easy to accumulate), then the labor cost is about $12,000. The parts cost is similar, hence a restoration of a typical $3,500 car is about $24,000. Fully restored fastbacks are able to be purchased for about $8,000 less.
Can you beat this price? Yes, but only if you have the tools and skill to reduce the labor costs, and only if you find a car with almost all original parts, a very good corrosion free body, with good chrome and stainless steel. You won't find a car like this for $3,500. You might find one for $8,000 if you are very lucky.
The $3,500 car will need everything. It will need corrosion work, new moldings, body work, engine, transmission and drive line work, suspension work and all new upholstery. It may also need all new weatherstrip and moldings, etc. The problem with starting with a car like this is it quickly becomes a three-year project and unless you have the stamina, it will never get finished. It takes about one-or two hours each evening for two years to finish a garage restoration project. Only one in five starts actually finishes up. Yes, there are cut rate methods of getting the job done, but then you have no return on your investment.
The better fastbacks run about $12K to $15K. If you look in this price range, you should find a nice one provided you know what you're looking at.
If you are a serious buyer of a classic car, think over what you want from it. If you'd rather drive it than work on it, think of the better, higher priced car. Once you decide, then plan to look for at least six months and travel 500 miles to look at each serious prospect. Do not give in to temptation. If it is not the right car, don't buy it! A bad or wrong decision is very costly!
Best approach is to join a local Mustang club, participate in shows for a year. Most clubs welcome new members willing to help with running shows, fund raising, etc. Most do not care if you have a Mustang, only that you have a sincere interest in them. Study the cars and models. Learn the good and bad points about the cars.
There are some real pitfalls in buying an old Mustang and you need a year of learning before jumping in. This is true for any vintage car. Of the vintage cars, Mustang is one of the cheapest and easiest to restore!
'65 - '66 easiest.
'67 - '68 next easiest.
'69- '70 next easiest.
'71 - '73 hard.
'74 - '81 very hard.
'82-'93 about the same as '67-'68
'94 to present...Ford support still pretty much available.
The level of difficulty is related to the availability of reproduction and aftermarket parts. I hope this helps. It is not meant to deter you, rather to inform you.