Recommend learning how to do your own tune-ups. They're really easy (and free!), and very satisfying to do yourself. The pages markh links will show you what you need to know. As for how much abuse your bike can take, I can't tell you. The only way to REALLY know is to break it then back off a little.

Nothing special to know about disks.

As for how firm, depends on the shock, your riding style, etc. They should at least be firm enough that you don't bottom the shock very often or at all.