BMW 325i's are very reliable cars when they are well-cared for. The information I'm looking at says that BMWs made in the early 90s require what BMW calls and "Inspection I" at least every 10,000 Miles, according to factory specifications. This service includes changing the oil and oil filter, and possibly the air filter.
What service you should perform now also depends on what was last performed by the prior owner. The best bet for keeping the car tip-top is to take it to a BMW expert and have them service your car based on what was most recently done and what it appears to need. The oil leak on the engine is definitely a concern.
I wouldn't recommend mineral water--distilled water is best, or at least clean water with low mineral content in an emergency, but maintaining the right coolant mix is very important. BMWs are sensitive, high-performance cars, and investing in maintenance will save big bucks in the long haul.