Vargas is the best Left back you can purchase without spending 60m pounds on a player like Clichy, Chiellini or the aging Ashley Cole. Marcelo is good value overall but he has bad defense stats and he is weak. Cant stand that.
Otherwise get 21 yr old D Criscito that can play both center and...
I bet your squad is an aging one, with Seedorf, Pirlo, Nesta, Guttuso, Ronaldinho and Kaka in their 30ies. Best use that money to buy up all the tomorrows talent e.g. Fazio, Veloso.