I think Ted has some nice implements but looking at the three choices of eqiupment lines offered the first one listed the house brand/build looks the best. The 6' model is listed for $1246 not the 7' model. Still it looks too light, runners (skids) too short, 3" box tubing is what I would consider to be the minimum.
My new plane for the x749 has 60" skids with smooth skids and high quality graderblades that are reversible and is 414lbs recently weighed after completion. This is for a 4' landplane. My larger landplane with 96" width, 60" skids and reversible blades weighs in at 984 lbs (recently weighed). If I had it to do over I would make the larger landplane with 72" skids.
In building these two landplanes I used good quality grader blades 6"x5/8" double edge with 5/8"plow bolts for mountings. Good blades are half the cost of the implement. The frame needs to be heavy enough to prevent any twisting and flexing, my 8' landplane uses 4"boxtubing with a .250 wall thickness.
I agree with Brian that weight is your friend but would add that this weight should be utilized to make the implement stronger without flexing at the same time.
Materials for a good landplane cost roughly $100/ft of width plus labor and consumables (welding supplies, beer and pizza for the helpers). Unless your helpers consume to much you wind up with a much heavier duty landplane for a few hundred less than a store bought model.