It's not possible to "overfill" a helm, but it will get messy
I topped mine up over the winter when temps were low, as the weather has got warmer the oil has expanded and weeped out the vent, down the console and all over the floor. There is a recommended fill level with most helms and as mentioned above you should consult your manual. Much more important to keep an above minimum of oil in the resevoir to stop air getting into the system.
Pump life should be long, however if the hydraulics haven't been maintained correctly then failier can be very quick, any contaminated oil will destroy the components in a hydraulic system. So the secret is to use fresh clean oil for top-ups and keep the dirt out. A visual of the pipework on a regular basis is also a good Idea (chaffed hoses, corroded fittings etc).
Yes, pumps can be overhauled, but only if you know what your doing, if not consult someone that does, steering failier at 40kts is a little bit scary
Again, for the oil see your manual or look online. But chances are it's an aircraft grade hydraulic oil that most Marine dealers will stock.
What make is the steering?
Hope this helps.