My last car purchase was a '87 300D that failed on the hwy 17 around Scotts Valley, thats a real test for a car with a bad radiator or water pump. The owner left the car with a mechanic who sent the head to be checked and it was indeed cracked, the #14 head of course. with 250K+ miles on it but the interior was real nice, just a little wear on the driver side seat bolster where the seatbelt rubs (common wear point) so I bought it for $450 and had it towed to San Mateo where I worked part time for a garage. I found another #14 head and it checked out ok so now the car is a daily driver (my wife drives the other '87). Oh, yes I put in about $3000 in parts including rebuilding the sunroof and I still need to shotgun the rear suspension
The reason I believe the heads on so many 603 engines fail is because the cooling system is somewhat marginal under extreme conditions such as driving in steep mountains and in hot weather with the AC on. If the radiator has some scale built up internally it starts the downhill spiral toward failure. If the aux fans do not work (as many do, because the thermo switch carries all the current to the fan!) or the thermo clutch leaks out of fluid and doesn't engage and cool the radiator properly (also a known high failure item) then the head cracks from overheating.
Any of the numerous versions of the heads will fail if overheated. Even though the later heads are strengthened in spots thats no solution for a serious overheating on an aluminium head. Gassers also have problems with head failures and some of that can be attributed to not replacing antifreeze on a regular basis, or using the worng type of antifreeze, according to a mechanic at the local MB dealership who said they stack dead heads up like cordwood. Plan on replacing as much of the cooling system as you can if (when) you get the car running again and it should prove to be a reliable, safe vehicle. I love em!