It depends, actually, what kind of adventurers they are, and this is why I'm so adamant about the difference between soldiers and other fighters. Mercs will have a tank at about 10th level. A 10th level soldier will probably have "borrowing privledges" for some kind of tank. A 10th level criminal or vigilante or even group of explorers will be lucky to get their hands on a tank, let alone keep it. It all depends on what kind of adventurers they are.
I think you underestimate how quickly Wealth awards add up. In a game I ran, the party decided to pool their Wealth, by giving their Wealth awards to a single character (rather than dividing it amongst the party). By level 7, they had enough to buy a tank. They didn't (probably because they lacked the feats to drive and shoot the thing), but they could have. By the rules, anyone with enough money can buy a license for a tank, then go out and buy one. Doesn't matter if they're civilians, military men, terrorists, or what have you. By level 7, they can get enough wealth to buy a tank. A tank that can be blown up by a few decent spells, or a couple shots from other tanks. If the PCs use a tank against enemies that are on foot (and who don't really have any means to easily destroy the tank), then the PCs get less XP by the rules.
it would be rather silly though to do such a change. gota make sure I don't spout too much bullshit, leaves a bad aftertaste...