Don't jump on cheap offers that don't include cable and termination. An U160 cable w/ termination is easily going to be more than $30 when bought separately.
And yes, these SCSI hosts are backward compatible. From U2W aka U80 to U320, all drive speeds can be mixed and matched; if you add a device that's UW or slower, the entire bus (including all devices that COULD go faster) drops back to UW speed, and then again you can mix and match from UW (40 MB/s) down to original 1st generation SCSI at 5 MB/s or even lower. The cable and termination packed in by hypermicro also support that fallback.
Those pluggable racks surely include 80-pin "SCA" drives, but should have standard 68-pin wiring on their backs where the internal cabling goes. Else you'll need 80-to-68 adapters as RicheemxX said.
regards,
Peter