I'm going to guess that the soft lines are the originals, and despite showing no signs of leakage, simply need to be replaced due to age. Any brands I should look for (or at least avoid)?

Probably will drain and flush the system at the same time.

Anything else?

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1991 300D 2.5 "Rocinante"
2002 Golf TDI "Teen Spirit"
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1984 300D
1966 Mustang I6 3sp
1985 Mazda RX-7 GSLSE
1982 Toyota Supra
1977 Datsun 280z
1971 Datsun 240z