These factors can't really be answered properly until we know if your fishing for summerrun/winterrun.....river size/river conditions and presentation style. All these effect your outfit for fly fishing, because presentation is the key to sucess in any type of steelhead fishing. Your rod can be anywhere from a 81/2 ft 6 weight dry fly set up to a 14' 11 weight shooting head spey rod with leaders to match depending on the situations.
If you can catch steelhead with normal drift gear you are ahead of the game because you can read the water and understand presentation.
I recently bought Flyfishing for Steelhead by Trey Combs. He has a excellent primer on spey fishing for steelhead to go with his stories from some of the greats in flyfishing. He points several common mistakes people make during presentation of the fly. (including some of mine) I would highly recommend this book and found it quite informative. It is currently the featured book of the month club at www.steelheader.net
Tight lines
Marty