It would be good to know approximately how many feet of track you have ; how many main loops are running and whether they are electrically connected or not; and possibly a diagram of your layout.
In general, I would recommend the Lionel 180w brick or the MTH Z4000 for power. You should not need a separate transformer for each block unless your blocks are extremely long (see above).
In order to run both MTH and DCS and Legacy you would probably want to get both systems. You will need an MTH TIU and remote (or Wi-Fi hook-up) and the Legacy command base and remote. The Fastrack switches are fine and you can run them off of track power with no issues as long as you install a 22uh choke in each switch to avoid any DCS signal issues. There is a thread on how to do this on the Forum. No soldering is necessary.
Basically, everything runs through the MTH TIU. You will need some special cables to integrate the Legacy system into the TIU. Again, there are threads on how to do this.
You should buy a copy of Barry Broskowitz's DCS Companion (3rd Edition) which will be invaluable on showing you how to hook everything up.
You also have the option to run everything (not Lionchief) via the MTH Wi-Fi, which would eliminate the necessity for handheld remotes.
No problem running Lion Chief with its separate handheld.
You should also know that newer Legacy, Lion Chief and Premier engines come with a blue tooth option and the new Lionchief 2.0 + can be integrated into the Legacy system.
Hope this gets you started in the right direction.