Extrusions for router rails

Some help please. I was considering building a large 4X8 foot 3 axis
router for architectural type stuff (doors, windows, furniture..) in
wood. Absolute accuracy is not important. .010/.030 is fine although
repeatability must be high. In other words it doesn't matter much how
big it is as long as the cuts line up. With this in mind I have the
following questions:

1. Are heavy square/rectangular aluminum extrusions straight and
accurate enough to use as rails without additional machining?
2. Have any of you seen/tried filling a tubular way with an epoxy
motor or some such to increase rigidity? Did it work?
3. Assuming aluminum rails which do you think would be better:
plastic (turcite/teflon) lined sliding ways or rollers?
4. Gates is selling heavy cog belting by the foot as an alternative
to rack and pinion gearing. Any opinions on this? Have any of you
seen this in use? If it works it's got to be the lowest cost
alternative.

Thanks.