Although historically there were a ton of expansion options for the A500, most of them are really hard to find now, and expensive. An A1200 will be easier to turn into a WHDLoad rig, but if you want to stick with the 500 you'll probably need:
Indivision ECS, to use modern monitors
MegAChip, to get 2MB of Chip RAM instead of 1MB
Kickstart 3.1
Hard drive. You can either get a cheap old SCSI drive for your 590 online, or get a SCSI-to-IDE (or SATA) converter and then a Compact Flash adapter.
RAM. I think the 590 has sockets for 2MB of Fast RAM. You'll definitely want that for WHDLoad.
I think there are also some more recent boards available that give you some RAM and a Compact Flash slot that plug into the 68000 socket. Could make things a little easier. You'll probably also want to replace the 68000 with a 68010 (or another accelerator) to get the most out of WHDLoad.
So, bottom line, what you're asking to do can be done, but it can probably be done much more cheaply with a 1200 (and the 1200 also gives you the option of playing AGA games).
