I think this is a great idea. I actually had a similar idea and am starting a project, but with a couple differences...
no configs - Mine will have a config that is can be overridden in game or that can contain Mod preset quests that can be disabled in config or in game.
all in game config - See above
no commands - Mine will include the option of doing things via command since it is sometimes faster.
Op pulls in block, possible 2 blocks high like a refrigerator - I am going to have a special item for this instead of a block however you can set a sign with a quest by hitting it with the special item.
right clicks block and gets a gui - See above
gui has 2 slots - Optional slots for extended possibilities with quests
slot one is titled condition, and a ghosted quantity of items is put there
slot two is titled rewards, where another ghosted quantity of items is located
op then puts a sign or more on the machine sides or near (not required by the mod) explaining the quest
player right clicks machine, end is instructed to fill in condition requirement until the reward becomes unghosted - Give the option to have an NPC give the quest (think like villagers but a bit more info for players than is provided by the villager gui)
at which time they take the reward and the condition self nukes - If this is a NPC than they would just give the item and leave/have a new quest/ or the sign assigned the quest is destroyed
completion is stored in the scoreboard
there could probably be an option where if op places an repeater in a slot, that it becomes a repeatable quest - Also the option to do this through commands since my mod will have some option commands for ease of use.
My mod will be available for 1.9.4+ once it is done.