This is hard! For a faction that has the rep for being fairly straight forward, Guild can be difficult to play compared to other factions. Hoffman for example is really good, but he's significantly better as an Arcanist. Nellie is pretty great in scheme heavy games, but is hard for a new player and can get worked in "killy" games. Lucius can be a fun choice, but does the best with some extras that break your 2 or 3 box rule. The Guild Steward is almost an auto take in most lists and that ties up one box right there. I also think Daschel is super adaptable, but requires a few more purchases than your criteria listed.
I know people don't love Perdita this edition, but her crew is pretty mobile. I'm not sure what box Abuela is coming in nowadays, but having her, the family crew box and some monster hunters can be pretty good and an easy to understand first crew.