Most zebedee nuts (aka spring nuts) are zinc plated steel and will be fine outdoors in many conditions.
However, if you live near the sea or are going to expose them to anything resembling a marine environment then you should source some stainless steel ones.
I've also been looking into the unistrut itself. I like the light gauge 1.5mm option as it's strong enough to hold anything I'm going to attach to it and will be a weight saving over the standard 2mm or heavy gauge 2.5mm.
However, I have only been able to find the 1.5mm in a pre-galvanised option which is not really suitable for outdoors. For outdoor use your really want the HDG (Hot Dipped Galvanised) but I can only find that in the heaviest 2.5mm thickness.
There is also the option for stainless rails, but I can't really justify the extra cost.
HDG steel should last a very long time outdoors. You can also paint it for extra protection if you want. There are specialised paints for this such as Hammerite's "straight to galvanised" range.
80/20 would be the obvious alternative. Light weight, strong, versatile but you also have to consider the possibility of galvanic corrosion as aluminium and stainless steel would be prone to this.
It wouldn't be an issue with the HDG rails and stainless fasteners though.
So many things to consider....