Upgrading drive train parts on a brand new bike is a little odd, unless you got a killer deal on the bike or group set. I understand wanting to mess with and improve it though.

Tourney is a great value and I'd personally be happy with it on a bike I wasn't keeping for the long haul, except ... I don't really like the 7-speed freewheel and the wheels they come on, and Tourney is a little fussier keep adjusted. Maybe that's just me. I'd be tempted to start with the wheel, an upgrade with a freehub, select an 8 or 9 speed at the low end and an inexpensive shifter. It wouldn't cost all that much.