Dropped-offset is legal for every division except single-stack (which is 1911's only, so you wouldn't be there).
For USPSA you need:
- A gun of 9mm Luger or larger caliber
- Enough magazines to hold at least 45 rounds of ammo. 55-60 is better. Certain divisions like Production only allow 10 rounds per mag so you'll want 5-6 magazines for that, but you can always do Limited which you can load up your mags to full.
- A holster for the gun. Needs to cover the trigger guard. Needs to be worn on the belt - no thigh holsters unless you're LE or military. No crossdraw holsters.
Also, not TECHNICALLY a requirement, but I'd highly recommend mag pouches. Everyone who is even wanting to be competitive will be using them but if you wanted to you can technically keep the mags in your pockets.
I personally started in Production and am currently dabbling around in Single Stack, but as a beginner I'd say most people are best suited to just try out the game in Limited division for a few matches. There's less restrictions and so you're you're far less likely to run into a "gotcha" that you might in something like Production which is more strict.
Like ny said though - try it out for a couple matches and then decide what you want to buy. Eventually after you compete long enough you'll find that you'll want accessories or modifications on your competition gun that you wouldn't have dreamed of before playing the game
.