For sale by owner real estate site

Can anyone suggest the best WPMU plugin or plugins to use for a for sale by owner real estate site? I am going to charge the owner to list a property, have 2 or 3 different levels (more pictures etc) and need to have the listings searchable.

Here’s the tough part (for me at least) I have one main site to do the transactions but I would like to have search capabilities and listing results on up to 100 separate web sites that I own. I know I can do this with iFrame or something but I need to be more Google friendly.

Any suggestions?