I recommend the Salsa Shaft. With a Brooks you may need a little more setback due to the shorter rails, and the cam adjustment makes it very easy to get the correct saddle angle dialed in. The Thomson Elite is also an excellent post -lighter and a little more $$$.

Saddle-wise I don't think springs are really necessary unless you're traveling really rough streets. I think you'd be better off running chubbier tires at lower pressure - also heavier (like a sprung saddle) but you get more traction in addition to cushioning.