Great system, I love mine.
If you're getting new backs you can get the Pro SD backs which don't use foam. Many old Pro S or Pro backs have dead foam which causes light leaks. You can buy foam kits to re-foam them however.
You can get dirt cheap new in box lenses from KEH too. 90 and 180 are great lenses, I like the 65mm wide angle too. I've been thinking about a 360 but its huge!