Another option is you can also have your MTH locos converted to TMCC (operable by Legacy) using equipment from ERR (owned by 3rd Rail). Will lose whatever sounds are in the MTH locos, but you'll have command control. Since you say you are not technically adept, having someone do these conversions is probably the best strategy. Not inexpensive compared with operating in conventional, but allows you to continue to use only the Legacy system to control all.
You could also just operate the MTH locos in conventional mode if you have a traditional O gauge three transformer. Would need to operate them separately (in terms of power supply) from your command Legacy/TMCC locos. No additional purchases needed for this option.