I just bought 2 Kato F3's at a trainshow a week ago, bought the digitrax decoder for them DN163K-- also and had both units converted, programmed, and running in less than 30 minutes. Atlas plug and play are supposed to be good also. The older locos need to have some frame modifications (generally) to accept a decoder in the loco body. This usually means milling a space to accept the decoder. Aztech is supposed to have the ultimate in modified frames. They exchange your old frame for the modified one (at a cost also.)
Steam locos generally need tender mounted decoders and the modifications to isolate the motor from the frame. However, the Bachmann Spectrum 2-8-0 can accept a small decoder in the boiler area. I purchased one (Lenz decoder installed) from Tony's Trains. Starts on speed step 1 and runs like a champ. It is not a speed demon but looks realistic hauling 7-10 cars.