My advice for A500 broadband: buy an old A2000 or better yet an A3000 instead
I hate to say, but I'd have to agree. Funny though, back in the mid to late 90's I built up a beefy Amiga 500 and put it on the net. i was looking for more ways to beef it up, and many Amigans back then would tell me to simply buy an A3000 or A4000. Unfortunately back then (~10 years ago) the cost of doing so was staggering. Nowadays you can pick up an A3000 on eBay for about $100, less than the price of an A530 usually.
Well let me back up. Do what you'd like. If you wish to beef up the A500 and it's for the heck of it, go for it. I've been meaning to do this with an A1000 forever, I'll finally given up (lack of time). However if your ultimate goal is a modernish system without breaking the bank, A3000 would be the better bet.