I've fitted the same type of adapter plates to put Cinquecento disc brakes on my 126.
I'd be very surprised if new 500's parts will fit and in any event they'd probably cost you a lot more than the ones listed on ebay - which will fit.
The Cinquecento/Seicento carriers then bolt on the adapter plates which in turn accept the disc brake caliper. On my car I had to fit a 5mm spacer before I could fit Cinquecento Sporting wheels as the hub increases in diameter as shown here...
As for the rears, I wouldn't bother with disc brakes for road use.