Create the member account on checkout

We want to offer a pricing page, then the user selects the price, pays, and gets a site in the network. I've been reading some old support posts about the user has to "register" first before they set up the site?!

The workflow should be
a. go to http://mysite.com
b. click for pricing
c. go to the checkout page
d. signup and checkout at the same time
e. get a 90 day trial w/ coupon, but set credit card to charge after 90 days
f. after 90 days, card is charged