I understand wanting to keep your Benz as Benz as you can. My Euro 240D also has a simple overflow hose that runs down the shroud and deposits any overflow on the road. I have been through many days in excess of 100 degrees with absolutely no problems.
I bought another Euro 240D recently and have robbed many parts from it. It has a very nice aftermarket overflow tank and the proper radiator cap. I have my eye on it, if I ever get into a situation where I need to, I'll add it ole' Silver.
If everything is in good shape on your 240D, I don't think you'll need it.
BTW, if you're not already aware, if you add one of these tanks you have to have the proper radiator cap. The non overflow tank type cap does not have the valve which allows the flow to and from the tank.
I take it that you got past your pulley problems and have your car back on the road.
'84 Euro 240D, 516K miles
'88 300E 5 Speed
Over 800,000 miles in