Personally, if the battle is likely to be lost (if I don't have 2x the tickets) I aim to buy a lot more sparingly than if I'm reasonably sure of victory. I also buy more wildly if I can't afford to lose a fight, or if I just don't care if I lose equipment. I'd also much rather let 200 extra arrows fall into the hands of the enemy than run out of arrows.
For me, though, the key is to get your players signed up as far in advance as possible, then purchase to their specifications. I am not a fan of the recent "hire whoever shows up on TS 30 minutes before the battle" trend. I'd rather get my equipment around the players I know are coming, their classes and their demands.
Overbuy on low requirement shields.
This is all last strategus information, I haven't run a major battle in a long time, but I think it generally holds. I would also say that getting the most expensive stuff for melee is not nearly as important as ensuring your ranged weapons have equipment. Buy battle axes and longbows over longswords and short bows. Its simply how things work in Strategus. In the end, player skill, ranged firepower, and ticket number will trump armor and nice 2h weapons every day.