The lenses are more expensive because they are worth it. As with a lot of things in life you get what you pay for. You can't beat German glass. I've had a Bronica with Nikkor glass and it was decent, but no where close to either my Rollei or Blad.
If you are doing any serious B&W work you will want 3 backs. One each for - + and normal development. If all you are doing is color then you could probably get away with 2.
The lenses I use all the time are 50mm, 80mm, and 150mm. I also have a 2x for added length if I need it. A closeup tube is very useful for flowers etc. I use mine with the 150 all the time.
If your buying older Blad lenses they are optically fantastic. Just don't put a UV filter on them and make sure you use an EFFECTIVE lenshade to reduce flare.
These are my experiences, your mileage may vary.