is this a good tranny, what are the specs on it

There are many variations of the torque capacity of the T5 through its run in the mustang. The high sides are the aftermarket and late cobra T5s. But basically, a T5 is a T5 and shouldnt be used in factory form past 300 lb/fts and sometimes even less. This chart breaks down the years and their torque capacity.
Hope this helps, Scot Rod
http://www.ddperformance.com/Trans ID chart.htm