I'm not sure why the motor cover has turned that color, should clean up I think. 830 is the top of the line machine, a sought after model.
The top of the free arm pops up, there is a spring action near the bobbin case, in the upper left corner some where. There's often tons of fluff and dust down there. Make sure you take out the presser foot and needle before you attempt it, the finish scratches very easily. All the discoloration along joints in the body is from oil, and it will clean up. I have started to polish my Berninas with resin type car polish, something that will leave a protecting film on top. Bernina and Elna have a relatively delicate paint compaired to others.
The top of the machine comes off too, it's hinged in the back. Do you best to vacuum /blow and poke out dust you see down there, then oil all points indicated by the red dots. The bobbin case pulls out easily, and there is an ring that will come down for further cleaning in there.
If it's sluggish, keep on oiling and cleaning, pay attention to all moving parts under the top lid, move levers up and dow, turn knobs and test sew a bit. Test sew the different stitch patterns. Start using it, and keep up a very tentative oiling and cleaning ritual the next week or so. Make sure you use the right Bernina bobbins, they make the machine behave better (I have tried others with arguable success, but these days I have loads of the right kind).
You have to find
the manual and read it, there's lots of helpful illustrations. It's a very nice machine, one of the best :- )