I've been working with GPS systems for the last 4.5 years, and would like to propose a solution.
There is a piece of open source (free) software called OpenGTS that is primarily designed for GPS tracking. It also has a secondary use which is monitoring of fixed weather stations, oil & gas wells, water levels, etc...
The properties in the database could be defined as monitoring stations, as described as the secondary use above. When listings are added to the database, their street address could be looked up using Google's Geocoding service, then the returned .latitude, and longitude could be inserted into the database to draw maps.
The system can also generate location reports as it is now however, you would need to add other info to the database i.e. price, number of baths, etc... I see no problem with adding that.
You would not need to buy any tracking devices, or other GPS hardware to make the system work.
I would imagine you are getting listings from MLS which would need to be imported, that too can be done without too much difficulty.
As an added bonus, the system will still work as a GPS tracking system if you need to do that too.
The system would require a server at your office that has an internet connection to look up the latitude, and longitude of the addresses. The server would be connected to your office LAN so the data is available in the office.
Please don't hesitate to contact me on Freelancer if you have questions.