Hey,
I wrote a "scratch-your-own-itch"-module that might help you as well. It's really simple, but gets the job done.

What it does, is, you can choose which roles can add products to their cart. It greys out the add to cart button if you lack privileges. Check permissions page.
Just turn off anonymous checkout, display a block with your sales pitch to anon. users, hide cart from anon. users and you should have what you're looking for.

Module is somewhat un-supported, take it or leave it
Regards,