burning speeds vs reading speeds shoudln't matter - I've read disks burnt at 24x on my 1x PowerCD before, it worked fine. However, with the older drives, you MUST use a CD-R disk, and NOT a CD-RW. I'd say you've probably used a CD-RW, which these older drives can't read.

You can swap any 5.25" 50 pin SCSI drive into that enclosure.