Just to clear up (sorry if you're already here), S/PDif (Sony Phillips Digital Interface) is a stereo digital audio protocol. It's sort of like the consumer AES/EBU (sort of). So no, you can't use it as an analog/instrument input. It's a little confusing because it uses an RCA connection.
In order for a piece of gear to have an S/PDif Output that means it also has an A/D converter before it goes out. I'm not all that familiar with digital guitar amps, they may have S/PDif Outputs, but can't speak to the quality if they do, and there's probably nothing decent under $100. Simliarly with mics, there aren't any (that I'm aware of) with built in A/D Converters and S/PDif Output.