Difference in housings?
 

Is there a difference in the S4 and S5 rotor housings? Also, is there a difference between turbo and NA rotor housings? Thank you!

Spark plug hole spacing and threaded embossment for the knock sensor and/or block heater in cooler climates on S5 and up. Turbo and non turbo housings are identical except for the exhaust sleeve and extra coolant hole on the turbo version.

Will I be able to run an S5 NA rotor housing in my 91 turbo motor?

Yes, just remove the exhaust diffuser in exhaust port.

Do i have to do anything about the coolant hole for the turbo? How will the turbo be water cooled?

In addition to the hole, there is also a counterbore for an O-ring. You'll need to get a machine shop to bore that pocket into the housing. That feature usually makes it tricky if you're anal about matching the configuration of your original setup. Check the 'For Sale' section. You might find a forum member who has exactly what you need.

Im used to FDs so I apologize that I didnt take the coolant port into consideration. The above poster info is correct. Sorry for the mis information.