It should be just fine if it was taken care of. You can probably tell if the oil was changed regularly by looking in the oil-filler hole in the valve cover; if it's fairly clean - great, if it's really grimy - not so great. The rest of the car you should be able to tell if it was taken care of by what kind of shape it's in. Also, what the person selling it seems like can say a lot for what kind of life the car has had.
Parts should be relatively cheap because of the age of the car and since they really weren't anything special.
I'm not sure why you're asking about that small of a year range though, since the style didn't change until 94. 87-93 are the same.
It is old and it was considered a bit of a speed car so it is highly likely that it has been driven hard at some point, but the care is the biggest piece. If it has been maintained well it could last forever. It it has bee ragged on and left to rot it will need a lot of work. Find a good solid one, preferably from a mature owner who knows what has been done and has good records for the car. As with any old car, things will break eventually, but parts are plentiful for those cars.
Ive owned 4 mustangs 3 were gts 2 were 12-13 second cars but the one i had the most fun in was my 89 gt it was a great car it had 120,000 miles on it when i bought it and did;nt give me any problems till upwards of 200,000 miles and like the one guy said the 5.0 motor is bulletproof from the factory i have a 96 explorer with high high mileage never done any maintenance on it and only had to replace the radiator so far and its got a 5 liter so for a fun dependable car get the mustang
Like has already been said it all depends on how well the previous owner took care of it. If it had maintenance then you shouldn't have any real problems, the 5.0 is an amazingly reliable engine; but if it's been neglected then you may run into some troubles.
Parts are plentiful and cheap for the 5.0, you won't have trouble finding any parts.
currently the parts for the fox bosy mustang are the easiest to find in the world!!
87-93, you have so much options.
they are inexpensive to maintain and are pretty reliable.
mine never broke down or anything.
depends what cond. its in of course though