That's a very interesting issue you've raised. We want to incentivise spending, but not make it so pay to win that free to play players will be completely powerless against bigger players.
However, free to play players have less loot to offer, if there's some sort of cost to attacking someone perhaps players will only attack people similar to their level to maximise their spending. For example, x weapons per unit used to attack, but make defending free? Maybe something like that could work haha
I am pretty confident that simply allowing unrestricted rapine of everybody by the best armed will kill your game. There needs to be a mechanism that enables smaller accounts to band together to resist warlords, or there will be no reason to even play.
Also, there need to be limits on the amount extracted via raids. Winner take all will simply guarantee ragequits. Consider making the percentage of loot from successful raiding tied to equivalence of military might, enabling new players to opt into gangs that can help protect them until they can grow some defenses, and you'll see that massive empires raiding puny villages will not be worth the time. If a player with 20k units can only extract 1% from someone with 200 units, they won't bother, if tackling a player with 10k units will extract 50% of their drugs.
Either balance or you'll end up with one player and no income at all.
Thanks!