I've done something like this in the past.
You can show the media uploader on the front end of the site as part of the property adding process and you can filter certain elements of the media manager to not show. So you can hide the gallery tab, you can restrict file types, you can even hide additional media data you don't need property owners to see.
Here is a gist with some ideas: https://gist.github.com/4460967
The last time I did this I used WP e-commerce to deal with the listing/management of products, discount codes and orders and then on the checkout success I implemented my own logic to translate purchases into a new post (custom post type 'properties') and then created an interface on the front end for users to enter property info and click a link to show the restricted media upload panel. Customers/contributors can then save it as a draft, preview it or publish it once they are happy and then it's live on the site.
Then I also had to add the facility to log in on the front end and manage a list of their properties. The site owner could mark a customer/contributor as an agent which would allow agents to add properties without going through the payment process.
Customers never went into the usual WordPress admin. If a contributor ever tried to access an admin page directly they would get redirected back to the front end 'manage your properties' section.
I had a call only a couple of days back to say someone wants to go ahead with a site very similar to this that I pitched for before Christmas, I'm just waiting for the deposit invoice to be paid before I think about making a start on it.
I've learnt a lot since creating the first couple of WordPress property sites. So this one is going to be a big move forward.
The one tip I will give if you're making a site like this for the UK market is make sure your data structure is compatible with the Rightmove V3 ADF specifications. It's pretty much industry standard and will stand you in better stead should you want to feed your property info to other property sites such as Rightmove.