If the only reason you're interested in supply buildings is for event quests and you want to be able to use rushes, Lv1 Cider Mill is a favorite for some - bit bigger than a blacksmith, but all supply productions so rushable, and can be used for current age productions as well. If you're worried about space limiting yourself to 6 is generally good enough - you can augment with temporary blacksmiths for lots-of-long productions, and do 4 sets of 5 minutes for "current age productions".
Sleigh Builder is what I preferred for a long time (and is thus still in many of my "main" cities. The advantage here is the 24 hr production can be deemed useful (15 goods) outside of event time. Sunken Treasure is another option (1 FP every 24 hrs or 5 goods every 8 hours). Magnum Opus is often considered the theoretical best of the size-6 event buildings (1 FP every 8 hours) for outside-event production - if you want to do 8 hour productions - its 24 hr is only a crappy pile of medals. The disadvantage of these 3 vs Lv1 Cider Mill is for many production cycles they cannot be rushed (only coin or supply cycles can use rush items).
The "problem" with using event buildings for those quests is that you need to keep them your current age. Which means a steady supply of reno kits is needed, or replacement versions from a selection kit. This is also part of why I recommended "only" 6 if you only want them for events. It's enough to not be annoying for those quests, but not so many to be annoying to keep stocked on reno kits.
