No reason for all aluminum, and frankly, as they're all aftermarket and expensive and seemingly don't always fit...not seeing the reason for one. My OE radiator started leaking around 185K miles. Of course, ymmv, but for me, it couldn't be much better. Same with OE ET (mine lasted 9 years)...my wp is going on 90K miles (graf, I think).
Don't mess around with the cooling system...it's too important. Also, the pulleys you have are fine...when you get to them, replace both tensioner pulleys and idler pulley and inspect the rest.
I'm so depressed. No one wants to come see me. :(
Mangofy your E46!