My advice is not to buy anything. Unless you drop hundreds and hundreds of dollars they will just increase the power of your opponents and the bots you face so you'll just stand still or go backwards.

Put it this way, Plarium always controls the outcome of the game and spending money, unless you max out, is never going to change that mechanic.

Hello! You can still make payments in any currency you are using; the funds will be converted automatically.

USD is one of the lowest currencies. You'd be saving money :)