Karin,
That is a great question and is a topic that I have been struggling with for 16 years.
My original intuitive approach was to make every site highly focused on a narrow niche in a large market place. This strategy has paid off in SEO at the expense of having a single place and methodology for managing the sites.
Recently I have been more inclined to publish dual purpose sites in vertical niches where there is little or no competition. These sites have sections for borrower - lender , buyer - seller, investor - home buyer, etc.
If the domain is too broadly focused on general terms like real estate, mortgages, investments, etc., it will be completely meaningless from an organic search perspective. The only way to get traffic to those site is pay, and pay a lot!
I spend a great deal of time updating and managing the different sites, not to mention the domain fees. Most of these sites appear on the 1st page of Google and many of them number 1.
I hope this information is helpful.