I think the only thing that was nonstandard was that the A500/1000/2000 had female serial ports where male ports were most common at the time. As for the modem getting you online it may be that nothing less than a 56k will be allowed on some internet providers. This is a problem because a 7MHz 68000 processor can't run the serial.device fast enough to access at 56k.
Otherwise the only problems you're likely to run into with an a500 is that the graphics chips slow down the transfer rate of the serial port if you crank them up to full resolution or color depth.
I wouldn't waste time upgrading to an A600 if you've already got memory and a hard drive for your A500. There's a couple of sources for the CSA Darrenger on EBay if you want to take that approach to getting more speed, but I think the best upgrades won't help much.
Modern Amigas don't get very far online without either the AGA grapics chipset or a graphics card. Since you already have a hard drive attatched there may not be a passthrough port for the types of card-slot bays that the A500 can access. A500 wasn't designed to be expandable but it was great in its day.
If you had an A1200 I'd be encouraging you to get an accelerator card and some memory and you'd be off to the races in 32-bits with 256 color graphics. A500s are only 16-bit and mostly good for old games. :-(
I'm sorry if I sound dismal but you can probably get the a500 online but don't be surprised if the web browsers don't excell on Amiga right now. At least since you're here you've probably got a PC that can run a decent browser like Mozilla Firefox so your a500 can be a good hobby machine.
