That such good news, Rosie does sound as though she's getting her confidence back now.
The smaller Karlie wheel, 20cm is the size you want, I'm not sure about wodent wheels but about 20cm is fine, not sure which wheel you mean by the robowheel either sorry.
You can buy plastikote from amazon
& a few other places it may even be available in shops like B&Q, I get mine here
just be sure to get the water based, it says on the jar & has a plastic lid unlike the non water based unsafe paint which has a metal lid.
I would paint the whole wheel, you can get away with painting just the inside but I find it makes cleaning easier if you've already water proofed the whole wheel & I do find sometimes pee gets on the base too.
I've never had a ham stop using the wheel so unless there's a problem with Daisy's wheel too I'm not sure why that would be.