Openlayers with OpenStreetmap seems to be your best free option, then you would add your layers with WMS (like Mapserver or Geoserver). You could also use google Maps with a license.
I have lots of experience on this since I've developed a number of sites, some from scratch with custom tiles.
I have not worked with geoserver but with Mapserver. You can't expect the server to render milions of details of features on real time. I'd say that the performance of these must always be enhanced by simplifying the data to levels of detail (according to the scale or zoom) and maybe burning base data to tiles.
Now I am working on Mobnetics Map
This uses Openlayers , OSM tiles, postgreSQL and Java The beauty of it is that it adapts to any GIS data (by the use of metadata) and then allows users to query the data. It is under continuous development and at the time has only additional data for Portugal (as test case).
The next thing I want to do is to offer to clients hosting on gis data that they can setup and update remotelly, so that clients can work on the site as their own.
It would be nice to have a client as you as my company develops the site..
Otherwise, I can also do a custom instalation of this site on your server and maintain it remotelly. We have done this to some clients.
best regards,
Joaquim Muchaxo
