Welcome to the Org!!
Im not a 7.3 guy but I will try to answer your questions
1. you defiantly want a custom tuned chip, I would recommend Tony Wildman. I think he puts his tunes on the ts chip
2. Exhaust, Intake, Custom tuned chip and I light foot should yeild decent MPG's Gauges are a MUST when running any chip!!!!
3.The Tranny's seem to be the weak spot in these trucks, Im not sure what their limit is. I will let someone else chime in on this.