Hi, is it Tom?
About decoders. If you start with a non-sound decoder then the number of functions that you need will be determined by the lighting needs of the loco. Most decoders will have the minimum of 2 for the front and read headlight. For the generation of locomotive you are using, that may be sufficient. Additional functions could be used to independently light number boards, or the cab. On more modern diesels. you might want ditch lights or rotary beacons, things like that. However for the most part a 2 function decoder will give you the motor control and the two headlights.
When it comes time for sound you have 2 choices of approach. The first is to replace the motor decoder altogether with a Sound/Motor decoder. The sounds don’t count as functions, so the number of functions will again be determined by the lighting effects you want. Most sound decoders will have 4 functions as standard, so most likely more than enough for your needs.
The second approach is to keep the motor decoder installed and add a sound only decoder, such as a Digitrax sound bug. This introduces some complexity, splicing wires together and then correctly setting up and running 2 decoders to work nice together.
Personally I would opt to replace the motor decoder, you can then use that decoder somewhere else.
In either case, adding sound to these locos is going to be a bit of a challenge. Both locomotives are going to require that you hard wire the decoders. The RS3 is going to be fairly tight quarters for a decoder, let alone a speaker and sound decoder. if the Spectrum is one of the split frame era models, it can be converted to DCC, but there will be very little room inside without some frame modification for the speaker.
I just converted a pair of Yellow box Atlas locos, one made by Kato, the other a Roco, and they both proved to be great candidates for DCC. Hard