You can have a static home page.
WP Admin -> Settings -> Reading -> Front pageThe sign up button in the home page could take the person straight to PayPal, too. The less steps, the more people will checkout.
Yes, the free version of s2Member is enough for this.
If the person doesn't complete the pay process, his account won't be affected: if he had one, he'd still have it, if he didn't, he'd still not have one. If the person is logged in when he clicks the PayPal button, his account will be upgraded after payment, if he isn't logged in he'll be taken to the registration page after payment.
I hope that helps.