425 is a good deal, especially with 2 lenses. I shot a Pro-S with 6 lenses and 10 backs for quite a while before I went to Hasselblad...
Lenses are cheap - look at KEH.com. I'd also suggest getting a Beattie Brightscreen/Intenscreen for it, makes the world of a difference in being able to see and focus well. I actually preferred the waist level over the prisms, as the prisms are darker, heavier and there just ain't anything like shooting with a waistlevel. FP-100C is beautiful off the 6x7 back. As far as the backs go, I wouldn't worry about light leaks - it's cheap to get them refoamed. Just order some foam strips, sharpen a chopstick and use some alcohol to scrape out the old foam. Then, stick in the new strips and you're done.
The 90mm is the standard length for the view, equiv to a 50mm in small format, 80mm in Hasselblad. The bellows make it nice as you don't need tubes to get closer than normal. HOWEVER, do rack out the focus and check to be sure the bellows aren't crinkled or have pinholes - that'll do more damage than bad foam!
I dragged my RB all over the country, with a 50mm, Sekonic L-327 and Fuji Acros/Provia. Heavy, but with a op-tech prostrap, it wasn't bad.