Rumor has it that as of today, Quake is not on the index of the german Bundesprüfstelle für jugendgefährdende Medien (Federal Department for Media Harmful to Young Persons) anymore. This means it would be allowed to advertise and sell it.
I only saw one source for this and I am not 100% sure if I can trust it, so I mailed the BpjM and will report the reply. Asked about the missionpacks and the different versions of Quake (PC, N64, Sega Saturn). We’ll see if I will finally be safe from possible lawful interruption. Psssssht!
Doom was unbanned recently so this sounds very much possible.