Can I do this and is Pro Sites the right plugin?

I'm trying to understand what plugins I need to accomplish this setup...

I plan to use WPMU to manage websites targeted to small business owners. There won't be any "free" level, they have to pay to have a website with us. I do want to offer additional plugins and premium themes though, including ones I write myself as well as ones like market press, which makes me lean toward Pro Sites from reading the documentation. Can Pro Sites not have a "free" level - in other words, charge on all site creations? I do like the free trial and coupons, and I'd like to utilize those - but not have a free level at all. Is this possible?

And also very important, this venture is based around simplifying things for small business owners - and the themes I'm developing are designed to be able to have a completely redesigned wp-admin interface - customized extensively to make things easy for business owners that have no concept of running a website but know they need to get online.

I can see that Pro Sites adds options into the wp-admin backend, including some custom write panels it looks like from the video, as well as the premium themes stuff in regular menus.

I do a lot of custom post types, taxonomies and meta boxes in my themes - are these going to conflict with Pro Sites? Will I be able to work around it with my themes? Or will Pro Sites force it's own admin theme and overwrite my customizations?

And if there's a different way to do this, or a different plugin I should use, please let me know.

Thanks in Advance!
Barbara