Another standard VGA cable, like you already have, with the separate RCA connectors would be fine. I’ve picked a few up on eBay and they have all worked okay for me.
Between the OSSC and Akura I’d stick with the OSSC. I have my Dreamcast hooked up directly to the OSSC with a custom VGA cable that has a built in 15/31khz selector switch. I have the Akura and it does not output 240p and 480i games. It also does not support line doubling that the OSSC supports for multiple resolutions. I also find the Akura shifts the image to the far right of the screen. If you do want to get a VGA box to accompany the OSSC (for 15/31khz switching), though it’s probably a little overkill for your needs, I would certainly recommend the Toro…