Well, since the A500 doesn't have a battery you don't have to worry about leakage. However, some 512KB RAM expansions have a battery on them, so you might need to watch for leakage there. Buy yourself a can of air and give it a good clean out. Though try not to breath it in. It is an aerosol after all.
All you need to fire up the OS is a Workbench 1.3 disk. Assuming you don't have a later ROM installed. As far as resetting it to it's original state, it pretty much already is. There's no internal harddisk that needs reformatting, or any flash memory that needs resetting. You just need a WB1.3 disk or whatever games you have and you're set.
Expansion-wise, I'd definitely be looking to upgrade the kick ROM to version 3.1. Though some older software may not be compatible. A ROM switcher can solve that, so you can switch back and forth between the ROMs. A hard disk expansion is a good thing to have, for obvious reasons. The A590 was a good HD expansion.