Roof up, it's a lot quieter and more refined than many hardtop cars I've been in. You'd hardly know it's a diesel, much less a soft top. You do hear a tent-like 'pitter patter' on the roof when sat stationary at traffic lights etc in the rain, but my wife actually lists this as one of her favourite 'features', go figure...!
Roof down with all the windows down, things are obviously a bit blowy at anything beyond urban street speed. I still wouldn't say the engine is particularly loud though, although if you're going for an S then you'll have a nicely tuneful exhaust note to listen to. Put the windows and deflector up (so yes to your second question) and all buffeting vanishes. We've done 100 MPH down a German autobahn on a sunny day in complete comfort. A nice little feature is that if you adjust the heating with the roof down, it will automatically revert to it's previous setting when you raise the roof, but it also remembers your 'roof down' setting and automatically switches back to it next time you drop the top.