Welcome to the club :)
You've made a wise decision on the S1! I have an '09 and love it.
I would build the bike with SRAM Rival (that's what I did) -- it is a very good grouppo, and for the money you can't beat it. That will leave you more money for a better wheelset or other parts like saddle, handlebars, etc.
With a 51cm frame, it will be plenty light -- it'll be a sweet ride!
Let us know your impressions once you get it built.