Mr George,
Sense you already own FasTrack, I would slow down save and purchase both DCS & Legacy units, if you have the money to install them at the same time, it's a lot less hassle. You are sooner or later going to want them both anyway. It's easier to install DCS on your FasTrack 1st, however with your FasTrack you will want Lionel FasTrack Wireless Command Control Switches, that can be operated via remote control, by a Legacy Cab2 or TMCC Cab1L.
I run both DCS & Legacy on FasTrack, and love it. Make sure you get the most up to date DCS & Legacy units, when you purchase. No need for a DCS AIU when running FasTrack with FTCC wireless Switches. Purchase only the DCS HHRC and the TIU.
Have fun building your new DCS/Legacy layout.
PCRR/Dave