Anon125: If you ask Bonners about the new or used products they have in stock, or about their current videos, they'll reply. I know that, because I've helped on one or two replies - I teach there!
Michael (keyboardcrazy49) makes his own videos, they're not made by Bonners so you should ask him directly about the instruments he uses.
To help answer your question about your old PSR4600: 1) have you turned on the accompaniment to split the keyboard? 2) if you have, have you pressed Sync Start to start the rhythm with your first left hand chord? If you haven't then the chords won't hold. The 'memory' for chords only works when the rhythm is running.