Well to start off, if you still have your old CAB1 system, you can reuse it, That's what I have on my layout and it works great.
To run your MTH PS2 & PS3 systems you'll need a TIU & DCS Remote. Both have been discontinued but can be found on the second-hand market. You can also use the TIU to run your TMCC & Legacy engines by connecting the Lionel BASE to the TIU with an interface cable from MTH (50-1032). If you want to retain some of the legacy features when operating them with the TIU/DCS REMOTE, you'll need a SER2 between the Legacy Base and the TIU. Lastly, you can buy a new BASE 3 from Lionel, and a CAB1L remote (almost identical to your old CAB1 remote) and use that handheld (or a CAB2) remote with the BASE3. The new WTIU from MTH can use the older the DCS Remote but it must be tethered with a curly phone cord to the WTIU.
I've attached a couple of graphics that might help explain how all of this hooks together:
