Can someone advise how you can set your website up with buddypress to just be one level membership subscription?
If you don’t sell multiple levels, there won’t be multiple levels. You can sell only level 1, and that’s all there be above level 0 (a.k.a the WordPress Subscriber role). And if you register users for free, they get the Subscriber role and that’s all that your users get.
About using s2Member with BuddyPress, I think you’ll be interested in this video: https://www.youtube.com/watch?v=9yiph5uqXgI
I hope that helps.