Am I being silly?

[brman]

In a Nutshell Yes.!

There is no way you can build a machine that can cut aluminum and brass to any reasonable degree from wood and with only a £500 budget.

I said this to someone else recently also. The plywood alone will cost you more than steel, The only place for MDF in CNC for lighting fires to keep shop warm and using as a spoil board. Don't do it.!

Well, to be fair, I did sort of expect that response ;)

Just to clarify though, I would not use MDF (other than for a spoil board). It is nowhere near rigid and stable enough. Plywood I thought might be if constructed properly but it sounds like you are telling me otherwise.
Anyway, leaving that aside, any comments on the use of supported rods vs linear rails and fixed vs moving gantry? Am I more on track with those decisions?