Hi Vital- Have you looked for a local granite kitchen top supplier or a stonemason. Granite tops come in 20mm, 30mm and 40mm and would make a great machine base (and they come flat!). Buying granite or synthetic rock is cheaper then making your own EG. Epoxy is very expensive. I would use a std concrete thread insert. This is either placed when cast or drill and place once cast. Do not expand them! Just bond them in. Backing plates are not really needed. Bolts are epoxied into concrete directly. Look up chemsets. If you cast a bolt in or use threaded rod to create a stud then you don't need the insert. This can be cast in or post drilled and cast. Look up any concrete insert type Ramset etc for how they do it.
I have made a couple of machines and have come to the conclusion that casting EG is the solution as well but I intend to make timber moulds and make epoxy alumina parts....cheers Peter