Once you have decided to go with a SCSI interface, the next step is to choose the devices. Whenever possible, you will make things much easier on yourself by choosing devices which use LVD, automatic termination, and SCAM (SCSI Configured Automatically). Most new devices fit this pattern. Also make sure that you consider where these devices are going to go, since you need to consider the total length of the cable. And check to see if you will need an adapter for an external device (50 to 25 pin or 68 to 50 pin).
Cards and Cables
Buy quality SCSI cards, PCI when possible, and make sure it will support the bus speed you have in mind (SCSI-3 U2W for example). I recommend Adaptec cards as the best on the market, but I have also heard good things about the Diamond cards. Also make sure you buy high quality cables. Granite Digital makes great cables and other accessories and is well known for their quality.
A SCSI hard drive is set-up just like any other drive. Depending on your BIOS settings, you can set the system to boot from the SCSI drive, or from an IDE drive. There is no problem in using a mixed drive setup. Seagate, IBM and Quantum are all well known for their SCSI drives.
CD-ROMs and CD Burners
A great number of manufacturers offer SCSI burners (CD-R or CD-RW), but just plain CD players are harder to come by. I use Plextor for both my CD and CD-R, but there are plenty of others out there.
Any good high-end scanner will be a SCSI scanner. Check around on the web to see what kind of results people are getting with the model you have in mind. I use Microtek and you can read my opinion in the hardware reviews.
Zip Drives, LS120s, Jaz Drives and many other devices of this type are available in the SCSI interface. You can add any SCSI device into your chain as long as you have not exceeded the number of devices for your card. These will not be U2W, so if you are using U2W hard drives, attach these to the other internal controller of your SCSI card.