Um, first of all, if you're buying a car, why don't you have someone knowledgable with you checking it first? Always do a compression test, and a few other tests, drive it cold, see how it shifts, listen to the rear, strain the alternator, etc etc.
But to be honest, even a badly beaten stang at 70k is hardly broken in IMO.
Listen to the motor, lifter noise is normal on all modular engines, but listen for taps, heavy sounding taps, however I suggest you have a mechanic or someone expert to check the car, I've never bought a car without driving the hell out of it, checking the oil, smelling the tranny fluid to make sure it isn't burnt(burnt means tranny is screwed/automatic only).
But to answer your questions, the clutch should be fine, I've gotten 170k on a clutch before. It depends how it's driven, a clutch can last a very long time. Usually the TOB go's before the clutch, unless you suck at driving.
Also, they're very easy to put a stereo in, however, if it has the Mach system, I suggest keeping the stock system, and have a pro install amps/subs if you like. Unless you want a really flashly 6 disc in dash player, btw there's lots of room to customize audio in a stang if you're willing to get ride of other stuff there. You can have a very nice head unit, and also a nice a$s EQ also.
Anyways, I think you should get someone to check out the car before you buy it, to be honest, just by the way you sound, I would have no choice but to assume you don't really know what to look for as far as a beat, no good car, you're asking the wrong questions, and neglecting many questions that need to be answered before buying "any" car.
Good luck. If you must, pay someone to check the car. If they wont let you take it to a shop to have it tested, by appointment and all professionally, then don't trust them, there's something wrong with the car.
Peace GL
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