your post reads like I could've written it .. I'm at the exact same stage.
from my own design backgroud i tend to think a 2:1 reduction, geared or belted, would be better than putting a bigger stepper directly on the rack. the belt will also serve to keep unwanted mechanical loads off the stepper if your brackets/assmebly aren't perfectly aligned (budget machine) .. no offense of course.
i'd imagine that chromed (hydraulic) rod would work well and cost less though they aren't hardened. if your gantry isn't too heavy, that is; otherwise the bearings will wear grooves on the surface.
i started off with the "T" gantry design and moved it under the table (over the sides) so i have as few obstacles as possible when loading the table. i quickly moved to an "I" shaped (make sense?) gantry with shorter (closer) supports. my 4x4 table actually needs to be 4x5 to give me a 4x4 cutting surface. i'm considering putting the stepper in the center, below the table.