Hmm... The 500 is quite rare - the 510s pop up all over the place (IIRC, that's what Sean has).
With the Casios just make sure it works before you buy - most need recapping by now and if it hasn't been done already, it will likely give problems.
Is there anything you haven't written articles about? ?
Helps being interested in most aspects of music, especially gear and technology. ? I haven't written about actually
playing the guitar, because that's probably what I know the least about...