MTG
You don't actually need to dedicate yourself to the MTH line completely for DCS. IMO I think you will get the most out of DCS, if you do, but if you like some particular conventional engines DCS can do that on a separate track (The Polar Express and Hogwarts Express are conventional for me.). If you have the money to one day buy the accessories to control Lionel Legacy and TMCC, you can control those. AND, if you really get in to it, many locos can be upgraded to PS2, so they are DCS controllable. The kits cost about $180 and professionally installed will run about $300, so you'll have to REALLY like a loco to do that.
Also look at every manufacturers rolling stock. For the most part, if you like a particular freight or passenger car, it can run behind any Loco, there are some lighting considerations, with DCS, but they are easily remedied.