What would make adding a custom idler and correctly sized ribbed belt easier or better than the stock bolt-on quantum water pump pulley? The Q pulley is part number 068121029A, the shims are 068121185B (4 or 5 will suffice) and the belt is 10x650. The pulley is still available new from VW for $60ish and the shims are less than $2 each.
If you don't want either AC or PS then the ABA pulley works fine...