It is my duty to inform you that you can't really farm iron very effectively until you get to the moon. A vanilla style golem farm probably works fine but screw setting that up, and the gravel -> iron nugget washing recipe requires a very large amount of cobble, which is nearly impossible to transport without brass funnels helping you do so in batches. However, you can make many small units that produce a couple ingots an hour and consolidate that into a respectable output. ideekay.