How reliable is a 5.0

[MiracleMax]

86-88 (except 88 up CA cars) are speed density engines - no problemo if your not changing the airflow characteristics to much. If your gonna heavily mod the rascal then look for a mass-air car.

Ok what kind of problems show up on this car Anything expensive?

Theres nothing expensive about the car and the aftermarket has replacement parts for everything but the roof and firewall. So the wallet is the limit when it comes to this stuff (which bolt ons tend to be of the cheap variety but offer good value for the money)

If your going M5 then you've licked the biggest problem in my book, the DOA, which is a good tranny in modified form, but a bit weak stock.

What should I expect to run on a stock used 87-93 GT 5 speed?

14's should be easy in the M5 car

My Civic ran a 14.7 with just the motor swap.I havent ran It with all my mods but Im sure I didnt shave a second off for alot of cash spent.
I take it that you swapped in a v-tech?

Also I know it asking alot,but what would the above said Mustang run with good headers,exhaust,Intake.I plan on getting those parts right off the bat.

Nay my friend, get gears and pulleys and a tune up first, then maybe some exhaust. The former because they are the best mods on a stocker and the latter just for the sound (provided your entry into mustangdom hasn't been tinkered with prior) Save the headers and intake for when you get a set of heads and cam. Altough some shorties would look spiffy. If your gonna leave the motor alone a set 1-1/2 primary tube headers will work fine with an x-pipe and some straight-through mufflers. (ie dynomax bullets). Keep the exhaust dia reasonable to maintain velocity throughout (probably 2-1/4 from the x-pipe to the mufflers and them after the mufflers start bumping it up if you want the sewer-pipe look)

I think 5.0 Mustang & Super Ford magazine did an artible on bolt ons. Get it and judge for yourself as to the effectiveness of the different parts.