I don't know if this has been reported yet, but I noticed that there is no wait time after buying something. In the same turn, i was able to buy an armoury and a Horseman unit and it did get both exp upgrades from the Armoury and the Kepast (unique Russian building).
So not only can you buy as much as you want in one turn, there isn't even that "wait until next turn" penalty to use it.
So not only can you buy as much as you want in one turn, there isn't even that "wait until next turn" penalty to use it.