128 GB is a limit of IDE. SCSI limit is 2 TB.
About which harddrive to use and how, you should first read the manual of the SCSI controller. It will tell you the requirements. If you don't have a manual, check
http://phase5.a1k.org .
A criterion I would look for is to avoid adapters, i.e. use a cable which connects to the controller without adapter and use a HDD which connects to the cable without adapter.
For fast SCSI2 it is recommended to use 50 pin connectors and cables only. Those 25pin external connectors which look like printer ports are unreliable at best.
And for SCSI in general proper termination of the cable is essential, especially if it is rather long. Again the manual should tell you which terminators are needed.