Hero Kid, ipodzens offers a
refurbed 5.5 video with a 240GB drive installed for a pretty good price if you want to avoid having to do the installation yourself (and making sure you have a the right backplate, etc.). Based on the model number, it looks like these units are based on a 5.5 80GB, which is slightly better than a 30GB as it has twice the RAM (should result in less frequent disk reads and therefore longer battery life).
As for getting rockbox running on a 240GB 5.5 gen, as of a few days ago it looks like it's super easy. 3.6 was just released and appears to have large drive support, so you can just follow the official installation instructions and it should just work.