With 850k gold, you can afford the high end stuff on this list
, unless you can obtain them from some other source. Against Devils, the especially important ones are Mind blank (to hedge out dominate), Death Ward (to block Energy Drain), and Freedom of Movement (counter paralysis from blasphemy). While you can obtain many of these from buffs, most of them are short duration buffs, and you really want these effects 24/7, since the Devil might win initiative/surprise you and hammer you with these effects before you can buff up.
Flight is also important (since many of them fly, and a melee Cleric is SoL if he can't), but Clerics do *have* the option of using their spells to gain flight, and you really only need it in melee (rather than 24/7 like the defensive buffs above). That said, 850 k goes a looonnng way, and you might just want to grab an item of flight anyway.