Lionel AEM7?

Lot more selections if you have both Legacy and DCS systems, opens up a lot more possibilities.  A complete DCS system is about the price of a RailKing diesel. I also find it enjoyable to have both systems, provides variety.

Sure, any MTH engine will run on conventional transformer power. You will still have sound and smoke. There won't be any compatibility issues with your Legacy system. 

You could also look for an Atlas AEM-7. Those came with a rather strange proprietary remote control system that was semi-compatible with TMCC. Unfortunately, according to some sources it generated a signal that interfered with TMCC and you had to install some component (a capacitor or a choke, I forget which) on any TMCC engine that was going to be running at the same time. you could always swap out the original control board for TMCC. The Atlas toasters show up on the Bay fairly often, at varying prices but I think you can get one well under $200. 

The Atlas locos came with a system called LocoMatic. I think it may have been made by Dallee, but don't quote me on that yet

It was a separate control box included with the loco containing pushbutton controls to operate the loco on constant voltage track.

I'm not aware of interference issues with TMCC. These could interfere with DCS signals, so a choke was installed between the pickup rollers and...everything else. This applied to Atlas O's passenger cars, and pretty much anything else with a constant-voltage lighting system, to the best of my knowledge.

Lionel did make an Amtrak HHP-8 in a separate-sale version that you could substitute for an AEM-7, that can be operated in TMCC mode.

It has smaller motors than the MTH model and probably shouldn't be put in charge of a heavy long-distance-style train, but a it should be okay with a half-dozen free-rolling Amfleets

---PCJ