"Should I buy the optional F# and Bb bars that are offered with your marimbas?"
My impression over the years is that lots of folks buy the extra bars to have them if needed or because their budget will cover that additional cast - and then never use them. The bars can take up space in a school closet for a loooong time! F#/Bb bars are handy in the Orff curriculum for the occasional songs in the keys of F or G, but the vast majority of music played on these instruments is in C or related modes with the standard bars. You can always order F# and Bb bars in the future if they are needed, so unless you foresee the specific need for those, it's OK to save the $$ and go with the standard set. If you teach the Orff curriculum, definitely get them.