Sounds good, you've clearly got an idea what's involved! A few more thoughts:

- Re second hand: if you could find some old 'big iron' for approaching scrap price - e.g. an old planer or planer mill with a table about the right size, that would be a great shortcut and get you a solid basic structure. But it's a long shot as you say.

- Re drives - I'd leave that until you've finalised the structure a bit more and know how much mass you're needing to move, and the details of any ballscrews you may be able to get.

- Re face milling - certainly not difficult but they charge a lot because the difficult bit is them having to pay for the large machine needed to do it... Is the affordable solution you've come across epoxy levelling?

- Re welding - you familiar with idea of residual stress and stress relief to prevent distortion over time?

- Re resonance/vibration - wasn't talking particularly about anything provoked by machine movements or ballscrews. It's the cutting forces as the flutes of the cutter hit the material which will cause the structure to vibrate/resonate.

- Re ballscrews/servos... you've probably already seen but this ebay seller:

items in linearmotionbearings store on eBay!

has a good reputation for ballscrews etc at good prices - might be worth getting a quote for the lengths you need to compare with your local suppliers. Given the size of the machine I very much suspect you will want two for the long axis.