Hi,
To be honest when looking at classic Mustangs IMO The number one thing to be concerned about is rot, whether it's present, hidden, or was improperly repaired in the past. Once I have a decent foundation,then I worry about the smaller details. The newer fuel isn't great with these older cars but it shouldn't be a deal breaker if you're looking to purchase an old car.
My findings with the newer fuel an older cars is that the fuel hose doesn't seem to last as long. However you can buy the good modern hose for $5 or less per foot and a stock 60's Mustang uses about 1 foot total .
Fuel senders and pumps seem to hold up fine.
Carb rebuilds don't seem to last as long but a stock carb can be Re-kitted every few years for less than $50, if you do it yourself. For the record on the carb stuff,I'm not 100 percent positive it's the new gas as most of the carb issues I see are on cars that sit off the road for months straight (like winter in the northest).
If you have a specific car in mind feel free to pick my brain.