i) mercedes has a lot of trouble with their ignition locks. If you are not able to turn the key (even not when you turn the steering wheel left and right as far as it goes), your ignition lock is breaking down. The inner part of the lock needs to be replaced (common problem with the W201 and W124) and if I may advise you: get it done ASAP. It is an easy job as long as the ignition lock still turns (5min) but it is a hell of a job once the key can not be turned any more (half a day+).
ii) loosing brake fluid is not normal... Take a look at the calipers whether they are leaking fluid or not. It is possible as well that the cylinders of the clutch-system are leaking as well (manual transmission only); this is a common problem for the manual shifted benzes as well.
iii) If you have raindrops in the trunk on the side panels, the rear window is definitely leaking. Take a flash light and look up through the ventilation holes in the top of the trunk, just below the rear window behind the rubber trunk sealing. You will probably see water drops there and even some rust...
Hopes this helps, and your english is clear enough...
with a minor 600+k