Just a comment on the couplers. I don't recall how the Williams couplers are wired up. If they are like the old Lionel coil couplers, they CANNOT be used, because they are grounded to the track. MTH DCS couplers are grounded back to the board. You'd have to use a relay or buy and try to mount MTH couplers.
You'd be ahead of the game buying an upgrade kit, which will have everything you'll need, lights, couplers, etc.
Personally, I wouldn't waste time installing a 5-volt board in anything, because they have passed their expected life and you could be wasting your time.