I have a 1986 Mustang GT 5.0 5-speed.
My Mustang has been remarkably reliable. It currently has 207K miles (I'm the second owner; bought it in '88 with 29K miles on it). The engine crankcase has never been opened. The alternator, starter and entire power steering system are original. I replaced the clutch at a little over 200K miles - and was shocked to discover it was still in good shape. The original brake drums and rotors went over 200K miles (yeah, lots of commuter miles). The transmission has never been opened - some synchros aren't as smooth as they used to be, but it still drives fine. Even my radiator and heater core (which is noted for failure) are still original.
I've put a few sets of struts in it over the years. The struts are reasonable to do yourself: The spring is mounted separately, so no spring compressor required.
Most of my paint is even still original ('87 was the first year for Ford's problematic clear-coat paints).
I have replaced a few non-regular-maintenance items: The water pump's backing plate began seeping at 125K miles; the pump itself was OK, but I replaced it anyway while dealing with the backing plate. The TFI module (ignition module on the distributor cap) failed (common failure; there was a class action suit on this, but mine failed over 100K miles, which was the suit cutoff). I replaced the fuel pump (which is in the gas tank, but still a reasonable DIY job).
Yes, '86 was the first year for the multi-point EFI 5.0 in a Mustang. The horsepower rating was dropped slightly over the '85 carb models (210 to 200, IIRC). The stock '86 heads are more restrictive than later years; if you're out for absolute horsepower, most upgrades don't make sense until the heads have been upgraded. I decided 200 HP was enough; since it was my only daily driver for a number of years, I didn't wan't to sacrifice reliability for horsepower.
I believe much of my good fortune with this car is due to fluid changes: I've always kept up with oil changes, etc. I switched over to synthetic oil after about 130K miles (BTW, no increase in oil consumption due to the switch).
If the car has been well maintained, I wouldn't let high mileage scare you away - but look for written documentation and try to get a feel for how the car was driven. I've seen far too many Mustangs that have lived a very hard life.
