You won't need to face your BB shell unless you're using a BB that has the cups that rest against the outside shell of your frame, such as Actiontec. Shimano BB's screw inside the shell. I'm assuming you'll be using a square taper... are you? Remove the BB and measure the spindle to get your length. It's prob a 68mm shell.
Whatever you chose to replace your cranks with you'll have to be sure the crank arm offset is close to what you have now, otherwise you'll be needing a different spindle length BB. If you're not familiar with replacing cranks take it to your LBS for advice. The correct size BB is important because of chain line, shifting, and general perforance of your drive train.
Do you want used cranks? Get a pair of Race Face used on ebay. The Turbines often come available a couple times a week. These cranks are super durable, light, and stiff as all get out... great performers. Your original sized BB may just bolt right up.
If you're doing this yourself, buy the cranks first and bolt it up to your existing BB to check clearance and chain line, then decide what size BB you need.