If you think your post was long, clearly you haven't seen some of mine.
A well-kept 302 can last well over 150k miles easily. I've had one (in a '90 Town Car) that hadn't even had the valve covers pulled off at 200k miles. My '84 is creeping up on 200k and (far as I can tell) it's the original motor in front of a rebuilt tranny; my '89 was supposedly rebuilt around 150k (and swapped from an AOD to a T-5 trans) and had the dog-snot beat out of it by a string of prior owners before I got it, and it's still pluggin' along at somewhere around 275k right now. So, there IS longevity potential in these cars if they're treated right ... and even if they're not, they can put up with a fair amount of abuse and still keep going (within reason).
Just as well, if some douchebag throws a 250-shot at it and beats the ever-loving piss out of it on a daily basis, or doesn't think to change the oil more than once a year, or has a bad habit of missing shifts and over-revving it, or things like that ... ANY motor would be lucky to even last 50k. The rest of the car usually will show how it was treated. "Clean" is a relative term; just because someone vacuumed/waxed it, hit the interior with some Armor All, and sprayed the tires with some shiny stuff before putting it up for sale, it doesn't necessarily mean the car was well-kept.
"Is it worth it?" Depends. Is it worth buying? Sure, if the price is right. Is it worth $x,xxx? Dunno. Compare it to what other cars of similar years and options and in similar condition are going for in your area. Kelley Blue Book and NADA can go suck it - they're meaningless numbers when dealing with cars over 10 years old, IMO, especially when it comes to sports cars/trucks. Is it worth buying TO YOU? That's something only you can answer, yourself.
Personally, I wouldn't pay more than $3,000 for a '94-'95 GT with 140k miles on it, especially if it's an automatic or a 'vert. That's just me, though. And that's a broad generalization. If it was a Cobra, or if the car had a lot of well-done mods, or really low miles, or some other kind of outstanding feature(s), then the price range might go up a bit, but not something stupid like $10k for an average-mileage car with a few bolt-ons and a blower. Just shop around and use common sense.