I would just try and keep as much of your old sand that is in good health i.e. doesn't smell horrible and is not black. Do a good saltwater rinse or two to make sure you get most of the nasty waste particles out.
For the future I would make sure and have the following for the SB in your 155bf:
nassarus snails x 50-75
sand sifting stars x 3-4
sand sifting cucumber x 2-3
checkered and qween conch x 2 each
sand sifting gobie (diamond or twin spots work well)
bottom swelling fish like dragonets, jawfish, etc. x 2-3
This of course all depends heavily on the deapth of the sand bed. I would go for a 3-4 inch bed with a mixed grain size. 20lbs 1-2mm width, 40-60lbs sugar grain size or slightly larger, and the remainder of your current sand. this allows for maximum suface area and diversity of critter habitat.
Depends on what you like. I have a 4-5 inch SB in my 210g with a mix of Southdown and many different grain sizes of Carribean live sand. I don't have any big grain sizes(over 1-3mm), as I find it to be non benifitial and not attractive. My sand bed is always clean and looks very white. I have tried many different methods in the various systems I have done and find this method to be the nicest looking and easiest to take care of.
Garrett