Most Linux distributions would be good enough for us. But this part of the system really has little platform-specific stuff, i.e. it's mostly "business logic" (if you can call it like that) and typical OS stuff which is well covered by boost (file I/O, threading, I/O covered by ASIO, networking).
Concerning GUI: we're currently assuming that we will not be able to come up with the kind of GUI we want with one of the available GUI libraries (like GTK). So we'll most certainly make the GUI Windows-only. If the decision was made that the GUI needs to run on Linux as well, I would certainly calculate with more than 25% for that (despite using a library).