I have a '65 mustang myself.
If you want to be cheap buy really nice hubcaps. maybe $50-$80 each.
new tires run about $100 each.
new upholstery could cost $200 to $300.
depends what kind of new engine you want (6 cylinder, 8 cylinder,etc...)
example: new 6 cylinder engine would probably cost an estimated $5,000-10,000.
a new transmission would cost about $700-$1000.
I'll bet it's a straight 6 and not a V6.
Your best bet would be to pick up a catalog or get on the internet (for instance, www.cjponyparts.com) and price out all of the items that you need. There are countless vendors specializing in vintage Mustang parts.
Once fixed up, it's a great ride.