Non ferrous is my bag. The furnace is 600,000BTU so its capable of doing iron. The issue is lifting that weight as I work with a crucible. I'm sure within the DIY casting, a cupula would be better as you can tap straight into the mould (if your good). I can get one offs done in a few steel/iron foundry's in Sheffield.
The popular misconception is that iron is easy. In reality its the hardest. Personally I'd use alloy or brass every time. Much easier and cheaper to do.
Casting weight is what ever I can lift. 30KG at 800C takes a bit of bottle though.