I only thought of using Miata stuff because I happen to have a Miata sitting in the garage to take apart and look at. It lined up nicely with the target size & weight of the design. Miatas use a 4x100 bolt circle wheel/hub, and there are lots of options out there for compatible wheels in sizes I wanted (has to be at least 13" to clear any decent outboard brake setup).
Beam and swing axles are just horrendous. I know, you work with what you've got, but damn they're bad.
4130 is great for suspension links if you're going to properly heat treat it after fabrication. Otherwise, it's not that much stronger than mild steel. This is also why I would not make a frame/cage out of chromoly. Unless you have some cash you're not going to post-fab heat treat an entire chassis, so it's much simpler/cheaper to build it out of mild.
My frame right now is ~80lbs assuming 1020 steel, with some brackets and a few extra tubes to be added in here and there once I do some stiffness testing. So it'll probably end up about 100lbs.