Nirth: Isn't there a member here who is involved in the game development? I remember he said he worked for IO Interactive. Maybe he can shed some light if he's allowed to. :-)
Anyway, I read at Steam that the game is always online and a few people mentioned that graphical settings are (hardcoded?) locked depended on your hardware even if you want to experiment.
GOG should bring Blood Money, perhaps 26 to 30th May this year to appeal to the 10th year anniversary.
It's not always online, challenges are locked to online mode though. Online and Offline saves are separate and you get kicked to the main menu when you lose your connection.
If you wish to resume you'll have to reconnect or start a new offline save.