Frank,
JD is giving you good advise, if you are going to expand your Legacy engine line down the road, you might as well purchase the full Legacy 990 base with the Cab2 Hand Held remote Control. Of course this all comes down to what you know you can afford, If you can afford the Legacy Engines you can afford the Legacy 990 Base Unit also. In my case I purchased the Legacy 990 Base before I had a Legacy Engine. I never did own a TMCC base unit at all. I simply added the Legacy package to my DCS layout, and it works like a dream, connected to my DCS TIU. IMO having both DCS & Legacy is the best way to run a layout. I agree 100% with RTR2, having both the Legacy Cab2 and the DCS Hand Held Remote control gives the layout a big time variety of control, especially with the wireless FasTrack Command Control Switches, that operate directly from the Legacy Cab2, IMO one of the biggest upgrades in remote control train operation ever. As RTR says you can operate a lot more than just one train set up in this manner, most times there are 4 or 5 trains under DCS/Legacy control, a P2 Street Car and a Conventional Bumper Car all running on the multi level DCS/Legacy Layout all at the same time, some even on the same tracks.
PCRR/Dave