I don't know that sunlight is your only problem in keeping a lot of
wine upstairs; it might not get sun but might get exposed to a lot of
temperature variation too.

Maybe think about storing most of it downstairs and only keeping a few
upstairs. I have several hundred bottles stored in a pretty small
space and most of this was put together with 1 x 12 pine lumber. I can
take a few pictures and email them to you if you would like. It's just
generic horizontal shelving but i broke it up into sections that hold
about 20+ bottles each, that's about a carboys worth. The vertical
sections add a lot of strength too. I keep the homebrew in there too.

Joe