You have asked alot of questions. As a newbie myself ,there is no way I can answer all/any of them, (with any real level of expertise).
From what I understand, the moving table design lends itself to a more ridgid machine increasing cutting accuracy and surface finish quality when machining steel.
With an MDF bed structure, it will be a little tough to do any flood/wet cutting of steel. IMHO dry cutting or light mist lubrication may be possible for soft metals (aluminium, brass), steel could be iffy.
You can "index" large and full sheet goods (MDF, ply) and cut them with a table extension. (you would need to move the peice as sections are cut)
"Track System" will need to be steel or better (supported rails, linear slides) if you want to cut aluminium and steel with any accuracy. There are many DIY designs out there, also CNCRouterparts has premade bearing carriages that many use very effectively.
Hope I have been of some help. But as a newbie, my knowledge and experience is limited. I'm sure the true "experts" here on the zone, will chime in and offer much better advice. Good luck with your research and future build. Remember to post lots of pictures of your build.