What I did with a MPC Milwaukee Road EP-5 I have was to buy a Post War Celebration EP-5 version (I found a PWC New Haven EP-5 at a decent price), and simply swapped shells. The PWC versions are fully equipped with TMCC, RailSounds, directional/constant voltage lighting, electrocouplers (both ends), and crew figures. The surgery takes less than 15 minutes, mostly just moving the TMCC antenna, which is simple. I then sold the old Milwaukee chassis with the NH body, which offset part of the cost of the PWC EP-5. I've done this same thing with a number of MPC/LTI era Geeps as well.
As Ted S said, adding an ERR AC Commander is another option. That may have better low speed control, but you won't get the RailSounds. You could add the ERR RailSounds, too, but now you're getting into significantly more expense.
