All I know is, my 1998 Honda Oddysey will cost 3200.00 to rebuild. Am I gonna do it? nope... Major service on it... slipping gone. Clutches may be worn but at that price, I will sell it instead...
As well, the 1996 Taurus I just sold today slipped very badly (all the time). I went to get a quote to rebuild and they laughed at me. Apparently I own the 2 vehicles with the hardest trannies to rebuild. It takes an "expert" to do it right. They want 3400.00 to replace it and close to the same to rebuild it. I serviced it for 320.00 (almost fixed it too, only slips if you drive hard) and just let the new owner know prior what the issue was. Sold the car for 1000.00 although it was worth more if I lied and said everything was fine. Lying has a way of biting you later.. so I avoid that...
As for a Supra? no idea... it is a very simple task to pick up that phone and call the shop and ask. I advise NOT to take it to a dealership... but to go to a shop that specializes in transmissions. If you lived in Alberta I could let you talk to a guy who knows them intimately...
I personally would do a major service prior to rebuilding if its going to cost over 1500.00. A clean transmission will drive far better than a dirty one and you may be able to eke out 10-30k on it before you have to really pay attention to it.
Also, from what I read (just read the lower part) your engine revs and the power takes long to get you moving... (like a low stall torque converter). Thats not usually clutch slippage, but is a faulty torque converter. Those are relatively easy to replace (if you have the right tools and a shop). Clutch slippage is usually when you put it into gear, it takes a second and then bangs into gear... or when your going through an intersection from a stop, you start out in 1st and then the engine revs high and then BANG.. second gear...
as for rebuilding an automatic yourself... its fun... but if you aren't mechanically inclined.. dont bother... lol. I will rebuild manuals.. they are fairly straight forward, but to rebuild an automatic is a little more complex.. and touchy... I have no idea regarding the Supra auto.. but I do know on the other 2 i looked at, after I talked to several transmission mechanics, I decide I would avoid that area. I dont do anything myself when I am not confident of success. Any time I have.. its cost me more than if I had someone else do it.
Those are my thoughts.. just remember.. I'm not an expert.. just an owner.