I have a drill press and also a Makita table-saw with a steel cutting blade. So I could drill and cut box steel.

How accurate is water-cutting? There is is place near my Mum's home that does it, would it be precise enough to do the end plates and bits for the gantry? If it cut the outside dimensions and made a few marks where I need to drill for the mounting of rails etc.. then that could work well for me.