You might want to first look into getting a DKB Kwikstart II to add a real 512K bootrom (1.3, 2.0, 3.1, etc.) to the A1000, so no need for the kickstart disk. Internally, the best option for just a hard disk has to be the ICD AdIDE with a 2.5" ide harddisk. You'll still probably just have the original 512K RAM, but all of it should be addressable with the Kwikstart board in there.
Of course, depending on the particular A1000, various external harddisk + fastram boxes designed for the A500 may work on the A1000. There are many of these. You simple plug the box in backwards in the A1000 (i.e. left to right, front to back). Since the A1000's 86-pin expansion slot is physically higher than the low-profile A500, use some books or something to prop up the side expansion box.