My boards have 9mm Birch Ply tops, with a framing about every twelve inches - the framework is four inches deep. I bought my boards as a kit, so everything was cut and pre-drilled and extremely solid. As your going to be in the garage you should seal the whole lot - top and frame to keep the damp out. You could paint it with PVA or paint, anything to stop the damp attacking.
Your track underlay is purely a matter of choice - many will say you don't need it - which you don't, but personally, if you want your track to look like the real thing, there is usually a "shoulder" of ballast either side of the track. I use 2mm cork - it was self-adhesive and came pre-cut for points and crossings, with metre strips for straight and curved sections. It does not reduce noise, once you have ballasted the track you will have more "running noise" than you did before. I would not recommend the sponge underlay that Hornby produce - it does give up the ghost in time. BB
PS: A couple of pictures of boards during construction.