I'm guessing the $100 sizing check is a fitting once you've purchase the bike? I've seen the prices range from $35 up to $150+. If you feel you're bike is "great" but not "perfect" a fitting would be the way to go. As for the seat, I've got a brand and type I really like so I tend to just stick with it. If you don't have a favorite seat, try the one that comes with it and then go from there. As for the bikes you're looking at - since I ride a vintage Peugeot and a vintage Trek - I'm not gonna be much help! Sounds like you're having a fun time trying them out tho - and I'm always envious of new bikes!......let us know what you bring home!
Life should NOT be a journey to the grave with the intention of arriving safely in an attractive and well preserved body, but rather to skid in sideways, champagne in one hand, strawberries in the other, body thoroughly used up, totally worn out and screaming: "Yeah Baby! What a Ride!"