In Night of the Raven, the donations to Adanos that you can do with Vatras are completely pointless, and in G2 Classic they're almost entirely pointless.

Quests that you should complete before you're joining the mages are the ones that are in relation with the initiation of the other factions, because some of them might fail otherwise.

Something you have to consider though, is the 100 gold donation at the Innos shrines, since you get different bonuses depending on which faction you are, or if you haven't joined one at all:
No faction: 10 times +1 strength, 10 times +1 dexterity, 20 times +1 magic, everything afterwards is +3 life
Mage: 20 times +1 magic, everything afterwards is +3 life
Note that you can only pray once a day, anything more is just lost money.