But check out the competition before you make any design decisions.
Actually use their sites to seriously look for a property that would suit your tastes. How easy is it ? Whats good ? What's bad ?
Keep it fairly simple to begin with and if they have a large amount of property to sell, any search needs to be forgiving to those that don't know the area of the country.
And charge them an arm and a leg to "design" it. This may be the only fee you agree with them up front. Then charge them even more to develop it, an unexpected extra, which they'll be used to charging/fleecing their clients. Then charge them more to get it hosted.
I mean how can you agree on a development cost before you have a killer design that they're happy with ? And again, how can you even begin to cost the hosting without agreement on the design and idea of the volume of data.
Oh, and there'll also need to be an ongoing support fee, for those changes and enhancements the site will require. Did I forget any licensing costs, ie. MySQL ? Rule 1 is make sure you get paid first.