The 16mb of internal RAM of the Empeg will most certainly not be able to handle a database of 250.000+ mp3 files, so I'll guess you'll have to look at alternative solutions.

AFAIK, no ICE mp3car product is designed for that big a database, so I guess your only option will be to install a regular PC in your car with 7 or so harddisks in it that runs as an mp3 file server. Because you're using a PC then, the expansion possibilities are endless. E.g. touch-screen control would be possible.
The only downside is that this project will probably require custom-coded software, and this will NOT come cheap.