I'm considering learning how to play. As a 31 year old with no real musical background other than a year of saxophone in 5th grade, how difficult could this be?

That really kinda depends on you. But...
I do have a few how to piano books that my wife never opened when she tried to learn piano for 20 minutes a few years ago if you want a deal on a nice keyboard and some literature.

This I could be down with. As long as we are talking keyboard and not giant piano. I have a 600 square foot apartment, and I need something that stores easily.

It will be a challenge. Hopefully your past experience gave you a good understanding of basic musical theory, if not it's going to be even more of a challenge, and IMO going to make it a lot less interesting for you.

One of the more frustrating things about learning for me was the music books that teach you. They make you play some of the most trivial and annoying music. When I started, I wanted to play Chopin, Schubert, not Mary Had a Little Lamb, but that's what was in the books. It was music not worth playing, and it took the fun out of the process, so I stopped playing.

But latter in life I decided that I was going to pick it up again, with very little knowledge, and borrowed one of my girlfriend's Chopin Preludes book and found a really short song that looked relatively easy, and then I found a version of it on the internet to know how it's supposed to sound, and slowly but surly I was able to learn it one hand at a time, then put it together, and after a few days I was able to play it, and from there I went on to more challenging songs. And if I didn't know what something was, I looked it up on the internet, or asked my girlfriend.

But despite all of the mistakes that I would make, it was never frustrating at all, it was a blast. And now I'm a half decent player but the downside to the way I learned how to play is that my technique is unbelievably fucked up, but I don't care, because it's not like I wanted to do it to preform, I wanted to do it because I loved the music.

So long story short, there is no right way to do it. You're doing it because you're interested in music and want to have a good time, so I suggest learning in whatever way you want to in order for you to still have fun.