If you want only a harddrive, look for either a GVP A500 HD+/HD8+ or a Commodore A590 (make sure it has rev.7 Roms). The GVP A530 is a very cool thing, but it's usually pretty expensive...
Or wait until the ACA500 from Individual Computers becomes available. This new board will use a CF card as harddrive, plus it has some extra RAM, and a 14MHz 68000 CPU...