Also for the physical prep I'd highly recommend focusing on HIIT workouts. Even if you're fairly in shape, if you're used to those sorts of workouts they can be pretty brutal. Try to do like 60-90 seconds intervals of exercises like leg lifts, burpees, bicycle kicks, Russian twists, and pushups with no breaks in between for like 10 minutes. That's kind of what your big evolutions that people get rolled for are going to be like.
They don't actually check the underwear you are wearing, so as long as it's not showing you can pretty much wear whatever. I wore plaid boxers most days except for PT where I wore gray compression shorts that were actually visible during some workouts. Black, gray, or white briefs or boxer briefs should be fine during PT.
I do believe they make you buy underwear there because it's an inspectable item for RLP, but you're not required to actually wear it. Just make sure it's folded properly and in your drawer for RLP.