I'd personally advise getting a taser or other nonlethal weapon, instead of or in addition to a firearm.
Pistols of some kind are best, insofar as firearms go, but avoid revolvers--you can't silence them. Bring up two specificish examples and I'll provide some knowledge. How many people here have ever handled a gun IRL?
(subtley raises hand)
What about a crossbow? That's silent. Well, illegal too, but we're a supervillain. Is legality really a big problem for us? Of course, I'm talking out of my ass here, I know very little about firearms.
Crossbows are completely silent, or at least effectively so, whereas silencers merely reduce sound. However, it would be easier to find bullets than bolts.
And of course, we could get both.
Sticking to legal stuff is a good idea for right now.