The plastic impeller pumps have been OK in my (very limited, low sample size). I have one of those in my 1983 760 that I installed over 10 years ago as an experiment, and have been running with no issues since then.
HOWEVER, plastic impeller water pumps on VW TDI engines are known to have a very serious and common problem of the impeller coming loose on the pump shaft and then failing to spin, which of course then causes immediate overheating and can lead to engine destruction.
The D24/T water pump impeller design is much more beefy than the TDI design with a lot more "meat" around the area of the impeller where it meets the shaft, so it might not be as likely to have this problem even when the impeller is made of plastic. But, all the same, since metal impeller pumps are not hard to find, the safest bet is probably to stay with one of them.
As Goteborg Vapenfabrik mentioned, Graf water pumps are known to have the cast impeller. Hepu pumps do too, at least the last time I bought one.
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86 745 D24T/ZF 345k lifted 2.5"
83 764 D24T/M46 155k
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