Since I have a chipped tooth from a piece of mortar that ended up in a pizza, mortarless ovens have always appealed to me- especially steel plate domed ovens. I've scrutinized every mortarless oven I've seen, both on this site and elsewhere, and while some have some good ideas, many miss the mark.
First off, use firebrick for any brick exposed to fire. I've seen more than a few mortarless ovens use regular bricks, which is a really bad idea.
Second, insulate- both under the hearth and over the dome. I've never seen a mortarless oven with insulation and insulation is critical for maintaining pizza baking temps.
Third. Weatherproof. Rain on insulation is going to be both a mess and, with a very hot oven, possibly even dangerous.
Needless to say, what I'm describing is probably a lot more complicated and costly than what you're envisioning, but I think if you want WFO pizza, you've got to take the extra steps.
Another thing that appeals to me about a mortarless oven is the potential to save some money. Someone recently posted a link to a great deal on mineral wool blanket insulation (for the dome). According to my calculations, I think one could insulate the dome for less than $100 with this. You can't put blanket insulation under the hearth, though, and board insulation can get really costly, so that's a bit of a wrench in my own personal oven building vision. I might be able to fashion a perlcrete with sufficient compressive force, but I'm not sure. I'm still working on that.
Weatherproofing the whole unit with some sort of structure has been another tricky issue for me- especially some sort of inexpensive structure that could be disassembled and moved. It would be kind of silly to have a brick structure that could be moved, but a housing unit that couldn't.
One thing to consider when working with steel is that heat accelerates oxidation. That being said, you do see angle steel quite frequently in mortared oven plans (such as over the door) and I don't hear of any complaints about the steel components rusting out. If you are using steel, though, I would stick to steel components that are easily replaceable, such as angle steel and steel plate.