Re: Membership fee
although not an actual plugin, amember can integrate with the front end of buddypress and wpmu. So I don’t think it should be so easily discarded. If you explored it more I think you would be impressed at how well it can integrate and provide a much more stable and feature rich membership package.
But agreed, if it has to be a wpmu plugin then it won’t work for you.
Ps- I don’t work work for amember 
