I like a small (I think it's scale 16) yellow cross for sniper. Dot is also popular.
HUD doesn't matter at all imo, other than I like a crisp and speedy class selection (irrelevant if you have class switch binds anyway).
Sens, as people say,is largely preference, but I think it depends on how you like to aim. If you aim with your wrist I think there definitely comes a point where it gets really tough to aim precisely and also turn to react. On Scout I find that this happens around 30cm. But there have been great snipers (and scouts) with all ranges or sens so I don't think the sens itself is important, more that it suits how you like to aim and feel comfortable with.