I think many older DC locos are worthy of being converted to DCC if they run well, you keep the instalation simple, and lighting is straight forward.
I have found the biggest issue with older locos is making room, especially for wiring to the lights, some times i have had difficulty installing the shell afterwards due to the harness when hard wiring in a decoder, add lights and it can get a bit bulky. You will learn to make wiring very neat and minimal.
I have added sound to DC locos such as P2K SW1200 and and Atlas S1 (HO scale) both turned out great but the speaker takes up the entire cab, I can over look it as it was all I could do at the time.
As already mentioned Streamlined Backshops has some great speakers available. TCS has excellent tutorials.
With steam you usually have the tender available to locate the decoder, speaker and now Keep Alive. With older steam I would ensure you have good rail current pick up.