WoW. lots here to digest..........
I have a few DC ONLY locos, such as old time 4-4-0s and a few others. Almost all my N scale locos, both old, very old and fairly new are DCC. I even converted brill trollies (Bachamnn & Model Power) to Kato drive with DCC and even a pair of Plymouth ind switchers.
I operate a MIXED DC/DCC system. Parts of my system are completely isolated & DC, some parts are already DC/DCC switchable, and some lines (like main freight and main passenger lines) are DCC only..... Sounds more complicated than it actually is. I have well over 100 locos I converted to DCC, with only a few with sound.
I adapted some cars to sound, a few box cars and a couple REA cars that can be used with passenger trains. One old time baggage & one old time box car is also converted to DCC sound. I make my own power trucks from std trucks.
I have a dual DCC system, freight & passenger mainlines, each with its own Digitrax Zephyr Xtra controller. (most of my DCC decoders are Digitrax as well). Most diesels are not that great for sound, lack of good baffle, I have done a few larger diesels. I have a new Bachmann steamer and a DD40x that came sound equipped & sound quite good.
I find for most uses, sound cars work better for me than most locos, especially diesels and smaller locos. When I want to add sound to a train, I just place the sound car on a programming track and set the sound car ID to match whichever loco I am using. MY Digitrax sound decoders have built in default/generic selectable steam & diesel sounds.
JD