Hi,
Among being heavily engaged in the Track&Trace market using GPS enabled devices,
I have built my own reverse geocoding applications servers using openstreetmap data to couple all those coordinates back to street names. I would love to know the specifics of your data but your source file isn't there anymore. I would love to know what parts of the world this is about. I've written well established software to do this already using 5 different datasources I can pull this from. Depending on concentration of your source data (continent/world/local) etc. This can be done really fast (or not). I've written classes to do this. The 50meter radius requirement might be a big slow down by the way as this pulls way more data from sources that do have limits.