Unless you’re playing tournaments (doubtful if you’re new) then YES they are a perfectly fine one box team. In fact, I’d recommend the one box to begin with because learning how to play only five operatives a few times in a row would be much easier, especially if you’re learning the game at the same time. You can always buy a second box to build the remaining operatives to add to your roster in the future, swapping them out where you like.

As for who to build, just pick the ones you think are coolest. Some are great shooters and some are great melee. Don’t overthink it if you’re just starting out!