Apple made 2 SCSI cards, the Highspeed one you listed that goes for $100-200 each depending on the day of the week, and a normal one (no cache) that goes for $50+ (and you have to have the C revision ROM for it to work on the IIgs).

I have the normal one, works fine considering the machine its connected to. On mine I can turn the external drives off and boot directly to floppy to play games (they are all floppy based and not HD installable anyway). With the HS you need to do some things on boot or you get errors.

Any SCSI card and 4MB make the IIgs a nice machine, if you just want to play games a stock IIgs with the 1MB RAM card will do nicely.