Jerome

Problem is, as with most things in life, you cant have everything!. CNC is not much different!. you have provided a great example of two completely different types of machine, that I believe, you would struggle to make one machine that covers both requirements.

In Short, plasma requires fast movements, therefore a light gantry probably with low gearing on the drive.
Steel requies very slow movement, with a hugely heavy machine (for the strength), and a very high geared drive.

Im not saying you cannot make one machine that does both. But I am saying, that a single machine doing both wouldnt be very good at either task. Just my opinion others may disagree.

For the record. If you intend to mill steel... with any decent results you would be far better off buying an actual milling machine and converting it to cnc, than attempting the semi impossible task of building a frame strong enough to handle the forces required.

Probably best, just outline what your intentions are... read some build logs... there are many... then focus a build (or purchase) around one partiular function?