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Doing so would allow a slip in conversion for people with empeg or rio car players.

With you so far...

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This would be an ultra low buck player that only supports MP3 files.

Okay. Lost me now. Why would you want to replace the empeg with something that whilst cheaper, is exactly the same physical size/shape and does less?

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another more powerful option is a more empeg like player based on off the shelf laptop PC parts.

There aren't that many off the shelf laptop PC parts. The only ones I can readily think of are HDs, miniPCI cards and memory. Everything else is usually part of a single motherboard that is custom made to fit into that specific laptop. I think you could just about wedge a NanoITX board into a DIN slot but it'd be a tight fit with the HDs and PSU as well.

The hardware is the "easy" part anyway. It is the software that will take you the time to get working well.