Celice:
I think there is a steampunk supplement for PF?
There has been at least one (Thunderscape), but the bigger question is whether it's on the SRD (generally, no).
League Historian:
2) Guns. Guns are cool. Guns are fun. They can also quickly become an issue because who and why would you use a bow or crossbow long term if you can use a gun? Sure,we could say they are expensive but I anticipate the majority of players being able to afford firearms. Stick to black powder weapons?
Pathfinder, mechanically, doesn't function well with advanced firearms. I'd be in favor of sticking with the early guns, instead. As for the "why", the answer could simply be that chemistry is different in the game world and one of the components of gunpowder is expensive. Even if you don't go that way, maybe they've only learned to make gunpowder using an ingredient that is expensive without realizing there's a cheap substitute, instead? Technological development can be weird that way.
Gunslingers in PF are really very focused alchemists more than they're "guy that uses a gun".
League Historian:
3) Where do we stop?
Wherever you want it to go, really. Magitech can explain pretty much anything while keeping magic important.