1991 500sl

[Sid The Kid]

Hi....recently I made my parents trade in their 1999 BMW for a 1991 500SL. They were reluctant to pay 18K CDN more after the trade in for an older car but I assured them it was a better car. However I feel guilty now because I made them trade a perfectly working car for a less perfectly working car. I'm hoping with your help I can fix these problems before they inconvience my parents.
Problem 1 - The water pump leaks and every morning when the car is driven the belts squeel due to coolant dripping on the belts over night. The temperature will rise to 100 and then drop below close to 80 from time to time. Is it crucial that the water pump be replaced ASAP? Is there a procedure I can follow to make the work easier. Any tips or problems to watch out for?
Problem 2 - the transmission will sometimes vibrate when changing from 2nd gear. Usually when going up hills. This vibration can be felt on the steering wheel aswell. Its a 4 speed automatic. Also, when driving the car aggressively the gears will thunk on the first downshift upon de-accelerating.
Major Problem 3 - The engine oil was never changed in this car between 1992 and 1996. The mileage put on this car between this interval was 30K. Then from 1996 and 1998 The oil wasnt changed again and 26K was put on the car. After this engine oil was changed at 12K intervals until we bought it. The car now has 112K on it. The car was switched to synthetic oil at 108K just before we bought the car. Is this a major problem like I think it is?
I hope someone can help me so I can these problems fixed for my parents. Thanks for reading my long, and probably boring, post.