To be honest, pistols are king indoors. They're pointy, very handy, and can get good accuracy. Stock, out of the box, they shoot just fine, even on propane. Take a look at other 'newbie' threads to find which brand you think suits you best.
For glocks, or the usp, KSC is a good place to look. Glocks are very simple to use, it's easy to find kit for them, and they are so very common to find parts for. And you have a whole range of sizes and styles and features to choose from.
As for the USP, I use a USP compact to great effect. It's incredibly comfortable in your hand, has a great feel to it when shooting, and does everything a modern autoloader should. Unfortunately, I have had trouble with the trigger breaking. I'm not sure if the new system 7 has fixed this or not. However, you can get a really cool USP compact with a threaded barrel, raised front and rear sights, in system 7 with a metal slide straight out of the box from KSC. As far as accessories are concerned, there are a number of options, but you'll have to look a little harder for them. The USP has become more common in the last while, as well, so part aren't as difficult to come across.
A TM Hi-Capa, however, is probably one of the best bets for a workhorse pistol. I've used one a little, but from trusted testimony from team-mates and other players, it's quite simply the most robust, most accurate, with a plethora of after-market parts. You wouldn't want to, or need to upgrade to metal slide until you've literally broken it. So keep that in mind when choosing a pistol.
However, outside of the pistol world, there are many options still available. An mp-5, or a p90, a short m4, or a shotgun work wonders in close corridors. The AEGs would have a better magazine capacity, which can come in handy. However, a lot of the time, up close, the first shot will determine whether or not you've won the fight. This isn't to say that prolonged firefights don't happen, but ammo capacity becomes an issue in longer lasting scenarios, and games. So keep that in mind, if you know what sort of games you'll be attending.
Good luck with the indoor shooting, it's a lot of fun. Get a holster, at least 3 total mags, some good gloves and full face protection, and you're set to go. You don't need much more to be effective indoors. Keep fast and light, and be smart about it!
- Fides et Amicitia