The rails are (obviously) the main addition to the construction.
I’ll take David’s advice and double varnish/cover the rail areas and overlap end covers. I’ve got some liquid rubber that I’ve used previously and this has proved effective. Although, I’m using a pent roof shed so would not expect to suffer from standing water. (Hope not anyway!).
Stewart’s recommendation is along the lines I was thinking. In my case I’ll try angle aluminium. (David’s comment on weight being the factor here). Those wheels sound good. I’d identified some similar but I’ll go with the 75mm recommendation. I intend to try six each side.
Thanks for taking the time to help.