It depends on the transmission involved, and whether you want to do it right the first time or by trial and error.
Speaking in general, you will need calipers to determine clutch plate thickness, and likely a dial indicator and stand to determine end-play, plus a tool for pulling the pump. You should also have the bushing driver set for your trans ($200-300), and any special tools. If there are any worn bores in the valve body you will need to buy the reamer for each one at approx $200, although you may elect to have this done or buy a rebuilt. You may also need a pressure gauge for setting the throttle cable, assuming there is one.
Handling the transmission is an issue too. Some of them are heavy, and all are bulky. Once you get the torque converter and valve body off you MAY be able to handle it yourself, but don't count on it. You will need a way to have it standing up to work on it.
The simplest trans I have done is the turbo 350, which needed a very simple piston compressor (in addition to the bushing driver set). On the other hand, I just finished an AOD which needed a complete set of Ford AOD-specific tools (which I bought on ebay for $300) plus a couple of others.
If you want to play with a junkyard trans and have lots of free time, you don't have much to lose, but I wouldn't suggest you have high expectations.