I play piano and keyboards and actually have a Roland JV-35 for General MIDI/GS sound myself (the GS section is basically the Sound Canvas SC55-mkII synth engine). I've had it forever and still like it. The synthesizer is great but [snip]
You are all set then, as that synthesizer will faithfully reproduce SCC-1 music (as the SCC-1 is an SC-55 on a card.) You will never have to buy an SCC-1 as long as you keep the synthesizer. SC-55mkIIs cost a fair bit as well, so if money is tight, stick with what you have.
In my opinion, people should stick ton external modules when possible, not only as they more flexible than ISA cards, and there is always a module that exactly corresponds to the capabilities of the important cards, but they are much more common and durable than ISA cards.
If you are building a retro PC, you only need one 100% compatible MPU-401 compatible midi interface, which generally are not as expensive as the cards with synth engines on them.