1.  Buy Advance Adapters bellhousing kit and keep the Toy tranny/t-case.
2.  The R150 is what came behind the supercharged 3.4L.  It will hold up just fine for the 4.3
3.  Tough choice.  The chain drive is plenty strong but it will cost you more for the extra adapter if you want to go duals.  Only other choice really is to find an R151 if you can, or use the chevy drive train.  You will not like domestic trannies after driving a Toy.  I don't recommend it.
4.  See above.  Domestic stuff feels like sloppy crap after driving a Toyota tranny.

The 94 is a CPI 4.3.  It is considered one of the less reliable 4.3 variants.  Can't reamember the exact issue.  Something that screws up with the fuel sending I think and pukes fuel all over the engine.  If the price is right I wouldn't shy away though.  You get a few extra HP over the other 4.3s. www.s-series.org will be able to tell you anythign you want to know about Chevies.