Any SCSI CD-ROM with a 50 pin connector will plug "n play.
There are other CD Players that will require connector adapters (and more) so be careful what you buy.
RPMs won't make a difference.
24X read isn't THAT much more noticible than 4X in reality.
SCSI is SCSI. Any SCSI device is backward compatible.
Just make sure when you install that the new CD isn't terminated, (the HD provides termination for the far end of the SCSI bus in a normal or stock configuration), and that the CD is set for a unique SCSI address (or identification #) . ID 3 is the usual address used for internal CDs.
Also make sure that the physical size of the CD fits the carrier that it mounts on . 99% of the CDs should fit, but there's always an oddball out there.