Ok I will try to explain the different systems and what they do
Lionel Legacy 990 - Controls all Lionel Legacy and TMCC features in command mode. Can operate conventional equipment from anyone with the addition a Lionel Powermaster (I think that's all you need anyway). Will not control MTH PS2-PS3 in command mode.
Lionel Cab1L/Base1L - Controls all Lionel Legacy and TMCC engines in TMCC command mode only (not all Legacy features are available). Can operate conventional equipment from anyone with the addition a Lionel Powermaster (I'm not sure about this with the Cab1L/Base1L and Powermaster). Will not control MTH PS2-PS3 in command mode.
MTH DCS System (TIU and Remote) - Controls all MTH PS2-PS3 features. Can control all Lionel Legacy and TMCC engines in TMCC command mode (not all Legacy features are available). A cable and Lionel Base unit are required for the Lionel TMCC features. Can operate conventional equipment from anyone through the variable channels of the TIU.
MTH DSC System - Around $280-$320
Lionel Legacy 990 - Around $300-$350
Lionel CAb1L/Base1L - not sure, but I think they are about $100-$150 or so less than a Legacy 990.
This should give you an idea of what each system will do. The required parts can be determined when you decide what you want to do. I have DCS, no Legacy so I am not up to speed on the Lionel parts. I have had a Legacy system ordered for quite a while and hopefully it should be here soon.
Others have given better advice than I can, but if you want to ever run MTH PS2-PS3 and Legacy/TMCC, both a DCS and Legacy 990 system would be needed to get full use of all the features from each manufacturer. That's what I am trying to do.
Here's a video explaining DCS/TMCC/Legacy from Eric's Trains He explains it better than I can. Maybe this will help you a little. Eric also has many great videos! Watching them can be habit forming - Subscribe and enjoy!