The place to watch for wear on the Webley, is the little link on the end of the connecting rod, between the barrel and the end of the piston inside.
It's a smaller piece than maybe they could have used, and it is the piece that I have seen take a fair few of these out of service.
I had a Plainsman long ago. Fun little rig, other than the cost of CO2 at the time.
I know that the Schimel has a fair bit of collector value, that Oklahoma is a funky looking one that I cannot recall ever seeing anything about, though.