First the warranty they offer is useless. the only things they are covering is engine and the trany. If you ask anybody most engines especially 3L will run for a very long time. The staff you should be worried about is the of the components, trust me there are a lots of them. My suggestion will be cover you basics , carfax, check with MTO. service history, previous owners. The test drive and inspection is a MUST.Clicking noises could be a injectors. Good luck.
If you still want more info. Check out LEMON-AID it's book, great read, You also can run the thought by e46fanatics
One more please be more specific about your question. Very generic question often are ignored due to complexity of the answer.
Let me be the first to welcome you to the forum. Good luck with a buy.
If you have any question do hesitate to ask.