A standard 2.5 inch ide drive (ala laptop harddrive) should be fine. Just be aware that drives beyond 6 or 7 gig can have problems with needing additional power. There are exceptions, and ways to get around it, but depending on your needs that may suffice anyway.
As for an accelerator, my 1st thought would be one of the ACA630 cards, but Im not sure if anyone still has stock (to be honest though Ive not really looked into it). I dont want to exclude any amiga dealers here, but Amigakit seems to be one of the bigger places these days. I believe maybe Versilla is still around too. Sorry I cant be more specific here, but Ive never really had a need to use these places and am just going by what I read on forums. Im sure someone else may be able to fill in some details here.
Additionally there's also Amibay, which is a nice resource for amiga gear, albiet often difficult to be 1st in line to purchase.
Depending on your needs however an accelerator may not even be necessary. Perhaps see how you go with an a600 with a small harddrive (should be able to get one for under $10 for testing purposes from somewhere) first and see if that fulfills your needs?