#1
Is it possible for me to get one bass, that would be perfect for all three?
If not, at least electric and acoustic or bass and electric.
#5
There's a local band I've seen a few times where the singer plays an electric guitar and an acoustic guitar through a Gallien-Kruger bass amp. They both sound pretty good and... I imagine a bass would sound good through it as well.

So using that as my reasoning, I'd suggest a bass amp of some kind. Without a budget to work with, I can't give much more information than that.