Can't offer much advice on the freewheel... I currently have an ACS crossfire that I don't really like (too much play before engagement) on one bike and a cheapo Dicta (better) on the other. Had a Shimano on a bike a few years ago and never gave my freewheel much thought then, maybe that's the best reason to go with one of those.

As for the ratio, that's ultimately going to be whatever works best for you. Personally, I'm running 48x18 now and like it. I can still get up to a decent speed and hills are easy enough.