Am I being silly?

[JAZZCNC]

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?

My advice is to price up the steel from a good steel supplier because I think you'll get a shock. To make a machine that is anywhere near strong enough and good enough will require a lot of high-grade plywood which isn't cheap.

Regards the linear rail question then I'd go with profiled linear rails every time over round type. That said I'm not advising you not to use them and in fact, if you do go down the plywood route then I'd advise you did because it will be easier like you said and the accuracy and strength profiled offers would just be wasted anyways.

The best advice I can give you is to forget any idea of building it then upgrade as you go along. The upgrade route never works as usually what you find or would like to upgrade to requires a whole need or different machine anyways.
If it's just to learn on then could say fine build it cheap.! But in reality, it's no more difficult to build it correctly the first time or in such a way that it's very close and not too difficult to improve if needed. Plus if it turns out that you need a completely different type or size of machine you'll get your full money back and if done correctly may even make a profit which you can put to the next machine.
With a plywood machine it will be worth nothing but firewood when finished with, components like round rails won't be good enough for the next build or correct size even, so very little of the machine will be reusable. So all that time and investment will be wasted.

Don't do it is my strong advice. Take a little more time and research what you need, ask questions so you find the best components to suit your needs and be patient don't rush to buy components or build it too quickly. Like most things that are built from the ground up there is an order in which it's best to tackle the job. It's also like eating an Elephant.? One small bite at a time so no rushing.