I've done several transmission R&R's in my time. The most recent was on my MB to replace the rear main seal on the engine. On a V8 car, which yours is, the work is harder, you've got exhaust system piping on both sides of the transmission - mine is a L6, so only on one side for me.

This recent job I had a lift to do the work, and taking my time, it was about 10 hours work all-in-all. I wouldn't want to do it on this car without a lift.

The work isn't hard, just tedious. You do need a good set of tools - long extensions with swivels, etc. Preferrably swivel-sockets, where the swivel mechanism is integrated into the socket. On my car, the transmission-engine block bolts were 17mm heads.

AllData will provide instructions on changing the modulator, I remember reading about that on my car.

As for the leaking cam cover - I'd again recommend AllData - it provides good information on the proper torque values for fasteners - seals like the one you describe will leak if under tightened (duh), but may also leak if over tightened. I have both a 1/2" click-style torque wrench, and a 3/8" click-style torque wrench (needed when the torque values are less than 30 or 40 nm).

Before you call in the calvary for the tranny replacement, I'd make sure that all of the externally replaceable items - like that leaking modulator, are in working order and that the transmission isn't over-full.

I have no idea what a good used transmission would cost, nor where you would find one, although others on the board I'm sure do know.