Help with categorizing blogs by "level" or "type"

I have tried several of the existing plugins to solve my problem, but I was unsuccessful.

I am using WPMU to host several blogs. I would like to distinguish between these blogs by "level". For instance, level 1 blogs would not be able to install plugins, while levels 2 and 3 would. Level 3 would have more themes available than level 2.

Additionally, I only want the site admin to be able to change / assign levels, not the blog users themselves

I can write the applicable logic to allow/disallow features by level but need some way to 'categorize' the blogs. Is there a plugin I could use (or a hack?)?