Best weight for a do it all fly rod

I'm looking to invest in some decent fly fishing gear but don't know where to start. I grew up fishing for bass and walleye with baitcasters and spinning gear but I want something to fish for smaller species like trout and bluegill. I do want the rod I choose to be able to handle the occasional larger bass but that wouldn't be my target with this setup. Should I pick up a full fly fishing kit or is it better to shop for the rod, reel and line separately and what weight rod should i be looking at for my needs?