Regarding the spacebard, I'm not sure that I know enough to say what it feels like.. it feels like the rest of the other keys. The various keys (shift, enter, etc) that have the white stabilizer attached to the keys all have an extra rattling noise (not very bad, but noticeable if you're paying attention). The spacebar does not exhibit that issue.
The metal plate on top is fairly heavy, and magnets stick to it, but not nearly as strongly as my fridge.. so... some alloy? But the keys are all mounted on the PCB, not the plate. The leads on the switches are all bent outwards before being soldered, so replacing the switches would be some work. Keep in mind though, I'm a newbie, so might not really know what's good or not.