@WinterHawk has it right; an INT 2 character will not need Theurgy, because s/he’ll have been sufficiently clever in getting the key info for the raid to succeed. As the raid description text suggests, “finding out the timing of the winter tax procession in advance will require clever, subtle inquiry,” i.e. INT.
If you have INT 1, things can go wrong… it depends partly on your own choice navigation, but partly on your rep established by prior choices, i.e. whether merchants and aristos all hate you. If so, the merchant and Ismene will sell you out. If not, they’ll be willing to help you.
And yes, with 0 INT the best possible outcome is failure to get the info in time to set up an ambush, and the worst outcome is getting jumped by Phalangites.
This is pretty much the only case so far where it’s a real disadvantage to have INT 0. Before the tax raid, INT was the natural dump stat, as there were raids where COM 1 and CHA 1 were useful, but nothing that differentiated INT 1 from INT 0.