Unlike engines, Mopar actually improved the 727 over the years so generally the new stuff is much better than the older "classic" stuff. A late 70's truck or van transmission is a pretty good core and they are typically less expensive than a muscle car core. Just watch out for the lock up ones since they used some special parts that you probably can't use in a performance trans.

I used to be afraid of opening up a transmission. The first 727 that I rebuilt took me about a week to do. I read the book, took some parts off, read the book, took more stuff apart, etc. The second trans took about a day to rebuild and then the third one took half a day. Most of the time is spent cleaning parts since used transmissions can be filthy. Not sure what a pro can do, but once I got familiar with a 727 I could put one together in a couple of hours if all the parts were clean. I'm sure a pro can do it faster than that. They are actually easier to put together than an engine since you don't have to measure as many things. Once you figure them out it is more like putting drum brakes on a car. Lots of springs and frictions and little clips.