It looks very nice. And as others commented, I like the idea of lawnmower knobs. Especially as I have one I need to get rid of this springs clean up day. I will strip them off before it gets loaded onto the trailer.
As for the sides, I think even paint would help seal them up. I try to follow what I hear Tommy Silva always say on This Old House, "paint all 6 sides of a board" to prevent any moisture from getting in.
MDF centers can really expand a lot. I have a version of a workmate table with the two top pieces having a melamine center. It got left outside not knowing it was going to rain. Instead of being 1/2" wide, I think the one corner was at least 1" if not 1.5" I never thought to seal the ends as it normally was put away every night.