Pete, the PS3 kit will have every item the PS2 kit had. The PS3 kit will be less the battery and related wiring. The PS3 upgrade I did was a PS2 kit with the PS2 board swapped out for a PS3. Everything will be present to do whatever locomotive you want. When Mike first came out with the PS2 upgrade kit, it was intended to do PS1 locomotives. It did not take long before the guys were doing almost anything that ran on rails.
I have done 148 PS2 upgrades and 2 PS3 upgrades. I have done Weaver, 3rd Rail, Atlas, and Williams. I am happy the guys will get the PS3 kits but more happy that all of us with PS2 5 volt boards will have a plug and play option to make those locomotives PS3.
I know MTH bashers are lurking but I want to say I know how much energy MTH has put into this program and being an operator I am thankful to MTH and the great people at MTH R&D.