User modify content like a wiki

I've a wp network installation but I manage all blogs.

I would like subscribers (I'm using buddypress too) could add post or modify what I wrote.
For adding post I use Gravity forms, and it seems a good solution.

To let my user modify I installed,
that is not too bad 'cos it send a compare screen of old and new version of the post,
but it starts give me some problem because sometimes I cannot change setting in posts allowing user submit his modifications.(if I check this box and update they uncheck again
Besides it let enter subscribers in the back end...

I don't use wiki plugin because I don't like the permalink, I can't associate the post custom fields ,like GOOGLE MAP PLUG-IN, furthenmore most of the content were already posted and I would not publish again with a new permalink.

Does anyone know a better way to shift my posts(and some page) user modificable like a wiki?

How much could it cost a modification like that from a wp programmer?

Or maybe someone has already experienced my problem and can give me some useful advise!