You'll get a buy button when you tap on the item, if you have enough gold. If you don't have enough gold, you'll just see the "INFO" and "CANCEL" buttons on tap.
I haven't figured out a way to do this one with the starting equipment without taking a hit:
For crowded levels like this one, when there's a choice, try to eliminate most of a specific kind of monster, which will lead to more safe spaces to take advantage of.
Throwing your spear where you can pick it up on the next turn is usually a good move. If you throw it further or have to move away and can't get back to it quickly, you're probably in danger.
The early elites are very hard until you get some extra gear, but the payoff is worthwhile if you can avoid losing too much health.
Cultists are easiest with the spear, but you may need to waste a turn bashing to get them spaced right - once you break up their formations you can often kill several in a row.
