while they are "old" tech, literally millions were built and other cars Ford made shared the platform and drive train. Parts exist to pretty much build a car from ground up without using any original car. That being said, most everything is available and reasonably priced. The wear parts or typical replaced parts are often in stock at your local parts store and pretty much anything else can be had at any one of the many Mustang specific parts suppliers. Mustangs are a great car to get as a classic for these reasons.
Maintenance is pretty easy. They do have grease fittings unlike new cars with sealed bearings so greasing the suspension should be done. Cooling systems are pretty simple so periodic flush and change coolant are usually enough. Charging systems often can't handle a lot of extra load from electric fans, stereo amps, etc. Some upgrades like higher amp alt are good investment. Adding relays to the headlights lighten the load on the wiring and provide brighter lights. But again maintenance is no different than a modern car. Brakes would also a good idea to update if they are drums. But if in good shape to start with, do not require special maintenance.
Some of the more sensitive stuff would be factory power steering. The power assist is prone to leaks. If properly rebuilt or replaced it shouldn't require special maintenance. The older engines weren't built with the same tolerances as today's modern engines so they do use a little oil and should be changed more frequently than modern cars. Fuel system, especially if it sits a lot, can require special care. The fuel system was not designed for ethanol fuel. It's corrosive and attracts water and it can attack regular rubber hoses. Avoid ethanol if you can buy 100% gasoline but if you can't there are additives that help. And stabil or similar product is a good idea if you put it away for winter months.
I'd recommend a proper front-end alignment per Opentracker specs or similar for better drivability on modern radial tires. Modern gas shocks, new springs and bushings, and modern rollerized suspension parts will help it drive more like a modern car as well as addition of bigger anti-sway bars and stiffen it up with export brace and Monte Carlo bars and sub frame connectors.
Of course there are options for rack and pinion and tubular suspension and things like that to bring very close to modern cars or even better if you go extreme but doesn't sound like your intended use would warrant that. If the car is already in good shape and has some options such as PS and power disc brakes it won't drive like a lumber wagon and won't be a dramatic change to go back and forth between your modern car and classic. I drive mine everywhere and anywhere and it has most of the ORP suspension options and chassis stiffening as well as disc brakes. It handles pretty good. I even have added electric power steering. With the updated suspension, tires and alignment it's quite a pleasure to drive.
You asked about options. The classic Mustangs were available with almost everything you can get on a new mustang except power windows and electric door locks. AC, PS, PDB, cruise control, tilt wheel, etc were all options. The problem is finding a car that had all that. They didn't build cars like today with pretty much standard packages so any number of combos could be ordered. Most likely V8 cars might have a C4 auto and PS. The mostly had manual drum brakes. Not so many had AC and/or tilt wheel. But stuff like that can be added if you find a solid car with most of the options you want. More and more people are wanting the classic cars with modern options so chances are you could find one that's already been updated. This is great as long as you don't care if the car is "original". It will all really depend on your budget. The more option, whether factory original or added later, the higher the asking price. If you are not a hands on person you will want to get as much included as possible. Often it's cheaper to buy a "built" car than to build one. You just have to settle for someone elses idea of what is cool. But if you find one you like, the right color, V8, drop top, etc. then it will be worth the extra up front. If on the other hand you don't mind buying a more base model and doing some upgrades on the weekends and can do a lot yourself, you might get a better car for less money but just needs AC added or disc brakes or something like that.
