ClassicCar

As faustmb says - Start out with the best you can afford. For the most part, it costs a LOT more to restore a car than it does to buy one that is either a 'survivor' or already restored. Models that were popular at the time of manufacture will be abundant (or at least readily available), and most likely will be very established and supported in the aftermarket.

Be careful - a 1968 Mustang (as in your example) is 44 years old! A LOT can happen to a car in that amount of time.
A turd with a shiney coat of wax on it is still a turd...
If you are putting SERIOUS money down, spend a little more $ - and if possible - have a '3rd party' licensed, certified (bonded?) professional inspect the car & give you a written report of their findings. This may be either be part of or separate to a mechanical inspection. If the mechanical inspection is separate, have that done also.
If the seller balks or objects, walk away - there are always many, many more to choose from.

Do your homework beforehand: - KNOW the car (make, model, engines, trans, market value, etc., everything);
- Line up those 'inspectors', have the names/numbers of firms or professionals that do this sort of thing;

It is well worth the time.

Also remember - Unless the seller contributes or pays for the inspection, it is yours. Don't share or give a copy of any inspections to them, unless they either share in the cost, or pay to have it done.
And, even if they offer to pay for the inspections - DO NOT let them choose or recommend the facilities or parties doing the inspection(s).

There's probably a lot more that can be added - however this is a good start. ;)

Well, you've aparently decided what model you want. Buy a spiral-bound notebook, and start by making a list of the year(s) you are interested-in, the options that you'd like to have on it, etc.. Most want the 'Fastback, Mach1, Boss 302, etc., so you'll find that those are quite a bit more expensive. You can get them with everything from a 6 banger to a 427 side-oiler (BIG BUCKS), automatic, 4-speed, etc.. Revise & update your list as necessary.
Go out to NADA.com and see what the going price is for one, in the condition you are looking-for. Compare that with online sales, auctions, etc., which will help you get a 'ball park' figure as to what you should expect to pay for what you want.

As far as books - Before you buy, go to your local library, look thru what they have. Contact a local 'Mustang' club (search the internet, you're sure to find one local to you). Contact some folks, go to some meetings - ask them the same question. If you ask 3 people, you'll get three answers.

If you are interested in working on your car, look through your library's technical area. Search Ebay for some technical/maintenance books - they will most likely have 'specialized' books on the make, model and year Mustang you buy. Ask those same folks in the 'Mustang club' what technical books they recommend for maintaining their 'stang' - and be prepared to spend quite a few hours listening and talking...