The velocity, is stereo, just had a look online
Basically, if you are buying an xt live, then you'll want a power amp to run it through. Tube power amps will sound better, like a mesa 20/20 or so, but they'll be VERY loud. Most rack power amps are all stereo, so you run the two stereo outputs from the xt into the velocity.
In a cab then I'd probably find a 2x12 with a stereo input, so you can choose to go mono or stereo depending.
Cab brand wise, I've always been fine with marshalls to be honest, and they shouldn't be too much. Of course it's always nicer to have fancy cabs, but most of the time you gig etc you end up plugging into a marshall if they're provided...
As for good gear, never buy cheap brand stuff unless it comes highly recommended. Cheap normally means it'll die pretty quickly and you end up replacing it all the time.