If I have a buff on a creature that conquers on Monastery of Hirana and a deployed Pillaged Armory, can I stack Monastery of Hirana and Pillaged Armory so I would draw a card and then buff the creature?

Yes — you get both: draw 1 and end up with the buff back on that creature. Both "when you conquer here" triggers fire off the same Conquer, and since you control both, you choose their order on the chain (CR 383.3.d).

The key detail is that Monastery's "spend a buff" is a cost-within-instructions ("[do X] to [do Y]"), so it's paid when the trigger finalizes — before anything resolves (CR 204.1.c, 383.3.b) — and the draw happens on resolution. So your creature's buff counter is already gone by the time Pillaged Armory's "Buff a friendly unit" resolves, which means the buff lands on it again instead of being ignored for already being buffed (CR 427.1.b.1). Order Monastery first anyway as the clean line.

One requirement: Pillaged Armory has to be deployed at Monastery of Hirana, since its trigger is "conquer here" — if it's sitting at a different battlefield, only its "when you play this" Buff fires. The Buff itself can go to any friendly unit anywhere, so it doesn't have to be a unit at that battlefield.