I have decided to make my table out of ply with MDF spoil board. I was going to have strips at least the same depth of my spoil board for the lowrider to sit on and I am unsure what the proper material would be for those. I have seen posts of people having the area on the non rail side wear down during use where the wheels contact the table. I was wondering the best surface was to prevent that. Melamine shelf boards are pretty cheap, but I am not sure if that is a good choice.
I am using beech plywood for the runner, it’s not been a problem in a lot of machine hours. 
I just use mdf.
Depends on if you’re filling your gantry rails with concrete or epoxy for rigidity. If so, you’ll need to support the weight with some precision ground steel square stock under the wheels (alternately, cannibalize the ways off of a lathe or decent sized mill).
This is, of course, just joking around. Don’t fill your rails with anything (unless you’re an honest to goodness, bonafide materials engineer who has a very thorough understanding of rigidity, elasticity, dissimilar material bonding, and are sure the time and costs are worth whatever benefits you might gain). And MDF is probably fine for a wheel bed. If it really stresses you, make it a separate, replaceable wear strip. Then you can even play around with other materials, if you get that particular wild hair…