I have the Trixie ones like this, have had it since before we got the rabbits (over 4 years now) and they've been outside all that time, and are fine. the only thing that has rusted is the chain on the door locks, but we have them cable tied shut anyway (we have the runs opened up to use as fence panels).
Oh, the joins aren't very good either, but that might be because we open and move ours quite abit.
Even if you used the ground pegs in grass I don't think it'd be fox proof - my two can each lift the panels! they work good for our purpose (supervised free range) but I wouldn't use as a run, unsupervised, not in a million years (that's what we originally bought it for).
Also, lastly, my two figured out how to open the door hatches! nearly had a frigging heart attach when tia opened it, Rupert got out (our garden leads down to a stream, and we have a well pond!, surrounded by foxes, cats and dogs!) I panicked, luckily, he is a scaredy bun so didn't venture too far (hence why now, all the doors are cable tied shut!!!!)