Right in the middle of my C4 rebuild right now. The trans ran great when I parked the car years ago, but even then it blew a lot of fluid. Bought Ken's Badshoe Productions video and highly recommend it. So far during my rebuild, I have found a damaged pump gear and a servo housing with galling. My clearance in the forward clutch is too great so I am getting some thicker plates. I spend $200 on a total rebuild kit from Makco. I got all new gaskets, clutches, clutch plates, rings, all bushings, that weird spring in the forward clutch, thrust bearings with selectives, modulator, and sprag kit. I had to buy about $75 worth of tools off eBay, a better set of snap ring pliers, and another $50 for solvents, trans oil for assembly but not to fill on completion, and brake cleaner. I haven't replaced the torque converter yet but will do so with a stock unit for probably $80.
I am rebuilding it the way a service probably would not. I would guess that some of the back yard deals don't include difficult to access parts like the o-rings in the bottom of the clutches and certainly no bushings. That was my experience after tearing into this thing and recalling that I had it resealed about 20 years ago. It is not all that difficult particularly when I have the laptop on my toolbox with Ken's dvds plugged in coaching me through the maze. I encourage anyone to rebuild there transmission based on my present experience.
I am not finished, so I reserve the right to deny all encouragement in a couple of weeks.