Re: Speaking of the Bass Guitar...
I played bass for a few years before picking up the drums, and recently came back to it to fill a hole in a band. I was amazed at how much I missed it.
I'm a big advocate for learning multiple instruments, and learning the bass will definitely get you in on the ground floor of all the chords and root notes and things that guitarists ramble on about incessantly. It's also a very logical progression from the drums, because it's percussive in playing style and very rhythmic. It's also much more portable than a drumset...
Just like drums, best bet is to buy secondhand. A good compromise between performance and cost is the Squire series by Fender, or the Epiphone series by Gibson. Don't worry about too much earth-shattering amp stuff yet; get a good solid practice amp built especially for bass.
A lot of stuff that was on the radio when I started playing bass in the 90's was simply the bassist picking eighth notes on an open E string - even less musically demanding than playing the 'boom-chick' beat - no fretting required. I looked into more interesting stuff to play (like hooking up with my high school jazz band and learning to read music written in bass clef). Fortunately, a lot of today's music is more complex basswise and you should be able to flex some chops-building muscle as you learn those songs. Tab for songs is available freely all over the internet. Don't be afraid to find out how to find thirds and fourths of chords and explore what can underpin the guitar most effectively for the song.
Have fun and good luck!
"Jus suum cuique"