Welcome to the dark side. With a tank that large, you are probably going to want to go with Pandinus or Heterometrus of some sort. Or you could try Hadrurus, I suppose, though they have more issues with communal living from what I understand. Alternately, you could try something smaller like Centruroides, but you are going to need a LOT of them to create a good scorpion density.
Assuming you go with Heterometrus (hint, hint ), you are probably going to want to create a false-bottom tank to help keep the humidity up. An inch or two of water, followed by maybe six inches of substrate, topped off with lots of hiding spots, a shallow water dish, plants (real or fake) and perhaps a waterfall would be great.
If you go with a desert species, you will want to put in several inches of sand with maybe some other stuff mixed in (Kugellager uses sidewalk underlay or something like that and says it works great). It is best to wet this thoroughly, pack it all in and then let it dry. Then all you need are some good hides and scorpions. Check out the enclosure thread for ideas, as both John and I have posted some pics.
As for size, 55 gallons is large, but not too big for multiple scorpions. It is good to give them a bit of room. If you do chose Pandinus, they might have problems with prey capture. The faster Heterometrus, however, usualy don't. In the end, if you keep your prey density high enough or hand feed, you shouldn't have a problem. Alternately, if you want to make a smaller community, you can drive up the 55 gallon to me and I'll trade you for a 10.