I'm casting another vote for a keyboard, but not a piano. If you want to be a whole band, orchestra or choir, synth-group or guitarist, you need an electronic keyboard - more specifically, a workstation or arranger.
Firstly, they're easier to play than a piano; the weighted keys on a piano are hard work for a new player, whilst the semi-weighted keys on a good workstation still sense velocity (i.e. the harder you hit them, the louder the note) but without the strain.
Secondly, they are portable, unlike a piano. They're not lightweight, but you can take one to a party, round to a friend's house, wherever. You can plug headphones in whilst you're practicing and they can be hooked up to an amp for full-on hall-filling performances
Thirdly, you can compose full pieces on them, save your music to a USB stick, to an internal HD, to the cloud.
Finally, you can sound like a god, like Vangelis himself, using two fingers, even when you're starting out! These things are instant gratification which means you're unlikely to put it away disappointed or frustrated. As you get better, a workstation has the sounds and capabilities not to leave you needing more. Starting out? Play chords with one or two fingers. Hit the D key for a D major, hit the D and the C# together for a D minor. More advanced? Hit D, C# and C together for a Dm7. Or play them in full...
Now then. These things have a bit of a rep for being owned by blue-rinsed retirees who do nothing but play elevator music on them. This is mostly true, but it means you can pick up an ungigged second-hand one in mint condition for a fraction of what they cost new when these players upgrade to the latest and greatest.
We've got a Yamaha Tyros 4 in our studio which we use for basic composition because it's so quick to get ideas out and recorded. It's then taken to a PC-based workstation where we use stupidly expensive samples to record the final track, but don't think the workstations sound rubbish - they don't.
The Tyros 3 is pretty cheap these days, and there are tonnes to choose from Korg, Roland, Yamaha and many others.
I don't know what kind of music you like playing or writing, but I reckon with your musical background you could be playing stuff like this in a few months:
And with a bit (perhaps a lot!) more practice:
And if you don't like any of those, then bear in mind these machines will do everything from Arabic to Chinese, Eastern European to African styles, rhythms and sounds. Or dubstep, or trance, or chamber orchestra or soft rock, 80s synth-pop or big band or... well, you get the picture.
Now hit up Ebay or Craigslist for a cheap previous generation keyboard and buy one
