I happen to be with a design firm that focuses on Real Estate but I think these points could be of interest to anyone looking for a website.
1. I would agree that it is beneficial to use a design firm that specializes in real estate. Building websites and build real estate specific websites are 2 different things. We have a software solution that we add into agent websites that we did not build and can always tell if the designer has not worked in the real estate web design industry before. Knowledge of what Realtors need, what is most important on the site, etc are important things to know when building a site.
2. If you do pick a firm that does not make a lot of real estate sites but you do like their design, just ensure that you tell them all of the elements that you need in the site. (ie. listings page, sold page, area info, mls integration, etc) - We have added our software into great looking sites, but the designer did not create a page to showcase the agents solds, etc.
3. I think that the design of the site should be built with the search engines in mind, but if your serious about getting leads from the engines, hire a person/company that is good at dealing with the search engines. A great designer is not necessarily a great search engine optimizer - and vise versa.
I hope that helps.
Feel free to contact me with any questions.
Thanks
Steve