How hard is it to replace timing belt? I'm DIY guy and like to try one.
Where can I get repair manual?

 

There is a haynes manual out there for the Passat/ A4 that is pretty useful. Also if your get your timing belt kit from blauparts you can get a write up on how to do it. they also rent the tool. Which engine? The DOHC 2.8 is easy, I don't know on any of the other engines. You can also get the Bentley Manual but it costs more and the engine is the same between the Passat, A4 and A6 as long as it is the 2.8. If you have a 3.0 or 4.2 or 2.7t it is pretty similar but you might want the bentley manual (more money).

 

The gold standard for Audi repair/maintenance manuals is by Bently Publishing (click here to go to the Audi page of their website). This is a pretty knarly task. I'd also suggest surfing quite a bit before doing this. Most people recommend replacing the water pump and tensioners while doing this task (they will probably need it and the parts are cheap compared to the effort of getting in there). Surf around and you'll find kits that contain all you need.

Good luck!