Hi Peter,

If you cannot stretch to linear rails at the moment, you might like to have a look at mechmate design.
http://www.mechmate.com
http://www.mechmate.com/forums/index.php
I'm not necessarily saying use the whole mechmate design as its a little dated, but it has a lot of features you describe in your initial post.
Lots of info for making roller bearing rails, rack and pinion, and other stuff

Personally I would always keep the gantry as short as possible. 3m will offer huge challenges.

Cheers