Not sure on which you decided (plastic or metal), but the plastic one works just fine. Assuming it's well cared for, it will last decades. I bought mine sometime in the '70's and she's still goin strong. I think I paid about $25, waaaay back then, so it hasn't really gone up that much in price IMHO.
Wanted to ask what you meant by "my brake booster is not great"? If there's a leak, associated with that, probably not helping anything (as you probably know). Wanted to mention the small o-ring between the master cylinder and vacuum booster. My 240D's brakes were "not great" either, until I found that o-ring completely missing and replaced it. Easy job, don't have to open the system.
Just one more observation. If you're concerned, in the slightest, about what anyone thinks about the car you're driving, a 40 year old diesel MB is probably not your best choice (unless, of course, it's a mint condition cream puff). Even at that, most people just don't "get it" about these cars....no matter what. Cheers!