I was actually contemplating using epoxy resin to cast blocks large enough to machine down. Stuff sets hard as a rock and cuts and drills pretty well. Would have to test if a 3018 can mill it though. I guess a lot would depend on the spindle and tooling chosen.
I linked to the one on Amazon only because their links tend to stay active longer. eBay was where I was looking though. And you're correct. Much cheaper.
And I would love to see how you modded the Z axis. The Z travel is IMO the one of the biggest cons to these machines besides the plastic bits. Of course, my opinion is based on very, very limited knowledge.