I'm happy with the 7-speed cassette for now. From what I've read here and elsewhere, the front derailer and chainrings may have minor spacing width issues with a 6/7/8-speed chain, so I should plan on buying a 9-speed chain instead and not use my current chain, right? Also, if there's a better crankset/BB for the same or lower price, please feel free to make suggestions.
I would tend to go with the 9-speed chain because of the FD issue. You will get more chain/cage rub than is desirable when your chain is too wide (happened to me). A 9-speed SRAM chain can be had for $20-$30.