The Legacy system will run all your TMCC and Legacy engines, so unless you never expect to buy and Legacy stuff, the #990 is the way to go. If you just want TMCC, I'd recommend the CAB1L/BASE1L, it's an updated version of the old TMCC system, and it's currently supported.
Since you have conventional, I'd consider the 180W bricks and one or two Legacy PowerMasters to run tracks in conventional mode.