Hi Shep, first and foremost I'd say test drive one before you buy. The 370 uses a single clutch auto and is quite slow changing compared to most of the double clutch auto cars nowadays.
To answer your questions, yes it will change down automatically even in manual if your speed drops too low for the gear you are in. It doesn't 'shift' to manual if you use the paddles whilst in auto, you can change gear manually using the paddles and then after a few seconds it reverts back to auto, if that makes sense. I don't think the shift times are different in manual to auto (but I could be wrong). I found mine to be a bit lethargic so had the gear change map tweaked which helped a little bit, but then installed a sprint booster which (for me) made a vast improvement and makes the car much nicer to drive.