Call me a stick in the mud but welded steel isn’t the answer either. All of my rigid steel cutting machines have cast iron bases. In fact, the only steel on these things is the coverings for all the electronics and stuff.
What you want is an ASTM A48 Class 40 (Dura-Bar anyone?) cast iron base. It is cheap. It is easily machinable. It has dampening characteristics which reduces ringing (thank you large graphite crystals). It is also very stiff and resistant to flexing.
You can get it cast using green sand and closed cell foam masters. Then get the critical parts machined on it like the rail and rack mounting points.
I have to admit I am a bit perplexed why small CNC manufacturers don’t use more cast iron. I think they don’t realize just how cheap and easy it really is.