For my NTSC PS1, if want to upscale to RGB can I buy an SCART PS1 cord, a box that changes SCART to YUV and then just use the YUV cord into my TV? How would I get sound in this situation as I know the box doesn't have the sound output as well?
You could do that or simply use a PS2.
If you want to use RGB on a PS1, or any other console, what you described is good. Make sure the Scart cable is indeed RGB (some, often the cheapest, may not be. In doubt ask the seller.) You can take sound either directly from the console if it has audio out, or at the end of the Scart cable via a small adapter. Or you can mod the box very easily.
The biggest problem with PS1 YUV, either through a encoding box or a PS2, is that some TV just don't like 240p YUV. They can take 240p Composite fine, but for some reason 240p Component isn't good for them. Unfortunately there is no other way to know but to try.