There are a few things to consider when upgrading a rear end.... You can just upgrade to an 8.8 rear end with 3.73 gears and a tlok unit for 750-800 bucks. This seems like alot but really its not that bad for what you get.
You get a very strong rear end that will last a long time, you wont really have to worry about breaking anything. The only down side is finding one for a 99+ mustang. I know where 94-98 8.8 rear ends are, the only difference is the axles will be a little shorter, 1.5 inches over all. This will make the rear wheels stick in more. Wheel spacers can fix this, some just dont like them, that is your call.
You can find one from a salvage yard, but it wont have 3.73 or 4.10 gears in it. If you want to build the 7.5 inch oem rear end you should buy oem ford gears and tlok from www.gefracing.com
(or where ever they are just really good and have good prices).
Dont go with another brand of gears, richmond are known to whine and Im not sure about motive gears, not enough info on them available.
The only difference would be possibly upgrading to a 7.5 auburn differential, this is stronger. If you ever plan to drag race the car at the track go ahead and get one of these, the stock tlok unit from ford's spider gears are very weak and will break before anything else. When you hear of a guy breaking his 7.5 rear end 9 times out of 10 its the spider gears. I personally broke one of mine in half not really doing anything super hard on the car...
So in a recap buy oem ford gears and an auburn differential for the 7.5 rear end if you plan to be aggresive on it, else just get the oem ford tlok unit.
For install you MUST get it professionally done, bad installs will make the gears whine and possibly something can break. When you go to get an install ask about backlash specs, pinion sleeve crush depth, wear pattern, etc if they dont have a clue or give you the run around try look else where. A good install is hard to come by... try to get a warranty that clearly states if the gears whine they will fix it and if they break in XX miles its covered. Some places try to tell you whine is normal and will go away... it wont so dont take that crap.
Install should be 200-300... pay more if its a good place though... much better to pay to much for a good install than to have your stuff break and have to do it all over again.
Gears = 180.00
Tlok = 180.00 Auburn = 50.00 more i belive.
check summit's prices.