Well, that's a good price for a T56. I think they're around 3g's for a new one. I would open it up and replace the syncrhos before I put it in a car.
850 for the 12 bolt rear end is very priciy, considering it has nothing in it. I've seen 12 bolt posi rear ends sell for 300 usd on overpriced ebay.
I don't trust automatic transmissions even if recently rebuilt, so I would only buy it as a core.
The 454 would be the cheapest. GM has a 8.75:1 454 with all forged internals for $5000 that is just asking for some boost.