This sounds like a great opportunity to start a small tool chest. Most of these items are DIY. Valve cover gasket you can get on line for less than $30. A screw driver and 10mm socket should be all it takes. 02 Sensor will require you get under the car (Safely) with a 17 or 19mm wrench. Don't worry about the other "possible" leaks until you get the rest sorted. Oil drips are still cheap. As for the battery, Sounds like a good time to head to Sears and pick up a batter charger for around $50. They are good to have around. With a few posts to the forum, you should be able to get through most of this for a lot less than the stealer wants.
Good luck,
Jeff in MN
87-535is, 88-528e, 90-535i w/ 105k.