Re: Speaking of the Bass Guitar...
Also a multi-instrumentalist. My main instrument is the guitar, I can also play the bass too but I really don't identify as a bassist. I'm a guitar player that can play bass.
As for buying gear I'd go for a used Fender of an Ibanez, Gibsons are great but can be pretty expensive. This way if you don't stick with it you can always sell it and break even. Any ol' clunker will work for a practice amp. Obviously you're gonna want something bigger if you start gigging though.
Research the model you're looking for and pay special attention to the width of the neck. If you have smaller hands you're gonna want something with a slimmer neck like an Ibanez. Go to your local music store and try some models out and get a feel for what you like, then go online and see what the construction is and especially the neck width. This is very similar to how I found my favorite model of drumstick.
Hope that helps!