The tone...
Gearwise: A precision bass through an Ampeg SVT.
The bass he used the most, I think, is a Sandberg 5-string precision with added humbucker at the bridge.
Almost all the Helloween classics are recorded with the Sandberg I think.
He also has been spotted sometimes with a Jazz Bass, but I'm assuming that's not his main thing.
Playing wise....
Practice, practice, practice
Markus is known for using every possible technique in metal.
I'm thinking about learning "Take Me Home" myself, because I think that is his greatest bassline.
It wont be easy.
Markus rulezzzzzzzzzzzzz, btw.
edit;
Recently he started playing Yamaha basses.
I think he got endorsed by Yamaha, but I'm not sure. He promotes Yamaha though.
I haven't seen any good pics, but I think Yamaha TRB.