If you want to plan for the future, I'd go for the Legacy #990 system. That gives you the maximum capability and will run any TMCC/Legacy purchases with full features.
With regard to the TIU channels, I'd only use the fixed channels with variable input power, I think you'll run into issues varying the input voltage on the variable channels. For those channels, I'd use the TIU's capability of varying the voltage using the remote.