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!