It's been my experience that the helmet should be the purchased first. Once the correct color of helmet has been chosen then work down in order of selection. Helmet, jersey, gloves, shorts then shoes. Only now is it the right time to purchase a bike! With the kit fully made up and adorned travel around to all of th LBS and try to match up a new bike. Do not forget to attach a mirror to the helmet while matching up to the bike.
It's a lot like matching a tie to a suit.
Mud