texasjeff - Greetings and welcome!
You might consider an aluminium extrusion setup. No Welding, relative ease in "errector set-style" assembly - and you're only limited by your ability to source the bits and pieces cheaply.
You could develp a machine that would be "reconfigurable" for future use as a metal machine. I haven't tried metal milling myself - but expect milling Aly won't be too difficult. The extrusion frames can be quite rigid - and if you back them with a bit of ply - "she ain't goin' nowhere." The trick will be setting on your design - yes moving gantry - has some advantages - look for all the ways to keep it and the router/spindle as rigid as you can.
Some recent designs on the 'zone have shown the Y Axis being wider than the working "bed." This is good - you give up a little floor space in the shop - to get the full width of the work surface. You'll quickly note there are heaps of trade-off here.
Sorry cant take ya up on the pizza-n-beer , maybe later!:cheers: Jim
Experience is the BEST Teacher. Is that why it usually arrives in a shower of sparks, flash of light, loud bang, a cloud of smoke, AND -- a BILL to pay? You usually get it -- just after you need it.