We've had thoughts about finally revamping our back yard. Life has
recently thrown us a curve that may delay that (again), but I'll ask
anyway.

I'm thinking primarily of an outdoor dining table; simple, farmhouse
style. I'd like it to be over-large, but I'd also like to be able to put
it in a shed for the winter, so I'd probably build two normal-sized
tables instead, with removable legs.

So firstly, is this a stupid idea? Will I simply be building myself a
neverending project; one that will need refinishing every year or two?
We live in NY City; a pretty "versatile" climate with high humidity in
the summer, and of course, rain. The table would sit on concrete. We'd
keep some sort of fabric awning/umbrella over it, but nothing that would
truly keep it from the elements, (except in the winter).

If it's not stupid, what sort of wood should I build it from? Are there
perhaps plastic "cups" that are designed to protect the bottoms of (say)
4x4s where they would touch the ground? If I were to use metal fasteners
(for the legs, plus possibly pocket screws elsewhere), would those be
problematic over time?