“The Galileo Seven” is a great choice! It really sets up the Spock/Bones dynamic in a way that will enhance the movies. I think that’ll be my fifth if I ever get the chance to actually do this - I’ve got a couple friends back home who might be interested, so maybe around the holidays.
“The Doomsday Machine” has a lot of cool parallels with TMP (not to mention Decker’s dad), so that would be a good one if you’re including the movie in your marathon; just don’t put them back to back or else it might get a little monotonous.
In addition to Sarek, I think “Journey to Babel” contains a lot of the background about how unusual it was for Spock to leave Vulcan and join Starfleet, which might provide some context for the Kolinahr stuff in TMP and some of the other Vulcan bits in SFS and the beginning of TVH. The character dynamics are much more important than background info at this stage, though, so I don’t think it’s essential if you’d rather show something else.