I too agree that the 4L60E's are just too difficult a transmission to start out on. If, and I say IF the DIY'er gets lucky and it's just a simple blown up part that caused it's failure, then a rebuild kit, an ATSG manual, and new parts to replace the broken one might be the ticket.
The problem comes when you find the broken parts, but don't have the experience to know why it broke. You simply replace the broken part and put it back in, then it runs for a while and breaks the same parts, or other parts, again. That is the perplexing part. The understanding why certain parts have failed is key to a successful build.
With the 60E's, you have to bear in mind that not all problems start inside the transmission. Remember you have a computer, wiring harness, sensors on the engine all that feed information to the transmission. Faulty sensors can burn up a transmission. Then if you go looking for the answers inside the transmission, you will be looking in the wrong place. Yes, you may find broken parts inside the trans, but you will not have found the initial source, and you will be pulling the trans again all the while cursing the transmission when in fact the problem has been external to the unit the entire time.
In talking with a friend (newfound friend since I started my first build) he admits that often times they are in such a rush to complete a job, and so short on time, that they will basically just throw entire assemblies (valve body, 3/4 drums, cases, pumps, etc
in an effort to solve the problem. He works at a commercial shop which I will not name, but they are VERY big. They are a large shop with hundreds of cores just stacked up so they can afford to do this. DIY'ers don't have access to all of the things that can make you, or break you when you are trying to troubleshoot. Having enough money to throw at a problem is the next question-do you have enough money to buy another valve body, or another case, or start throwing money at sensors hoping to figure out where the issue is??? I don't.
After having completed my first build (still not successful-no 4th gear!) I can say that it's not how a transmission goes together that determines success, it's the builder/installers ability to troubleshoot problems that equates to success. Even the BEST builders still sometimes experience issues with the transmissions once they are installed in the car (unless they are dynoed first). It's the knowledge of knowing what to do to fix those issues that is worth a LOT of money.
After just going through my first build, I'd rate a 10 on every question except rate of success with DIY'ers.. That one, probably <50% (if 6 months is the goal) I think short term anyone could build a unit to last a few months, but the true test is how long/far will it go.
Also, my hats off to you builders for I now realize just how difficult your jobs are, or can be. I'll never underestimate anyone who can rebuild transmissions for a living.