Suggestions on mechanics for thieves guild run by a PC

tldr; how would you mechanically solve information gathering and/or opposed rolls for a thieves guild run being run by a PC?

I'm running a Sword & Sorcery game, and one of the PCs is in the process of building up a thieves guild/information network in the city the PCs live in. I have assigned them a d6 resource die to represent the guild for some tasks (sometimes adding bonus for Streetwise edge).

A small group of thieves and assassins have entered the city looking for some artifacts the group have. How would you handle giving the PCs thieves guild a chance to dicover this other group? The enemy is very skilled and careful, so I thinking of a Stealth roll from this group opposed by the PCs guild. Would you just give them a notice of d6 and make opposed rolls? Would streetwise apply? Would you do something completely different?