@martin54
I think perhaps you've convinced me to persevere with the MDF one, although a nice strong metal one is still my ultimate goal. I want it for cutting plastic and wood for custom electronics/computer cases as well as model making. Although I'm sure I'll find the ability to machine aluminium handy as well. :)

@Jonathan
I think I'll stick with aluminium extrusion, since it seems a lot easier to work with than steel.

Thanks for the help, both of you. I'm going to gradually buy bits for metal one, now that I know the design is sound. :D