In terms of purchasing heroes, the fifth and third installments differ significantly. In the third game, the price does not increase when buying a new hero. Moreover, in the fifth game, resource piles are usually protected, unlike the third game, so some weak hero won't be able to take them. Therefore, in the fifth game, it is not advisable to buy a whole bunch of heroes at once. You can always buy additional heroes to create a chain and to transport the army.