Normally the first tip for new 340 drivers with aspirations of maintaining the car themselves, would be to become member of the club and visit as much garage-days as possible, where you can learn to do a lot of maintenance under the supervision and guidance of more experienced mechanics... But as you are from portugal, even visiting one garage-day will cost you more than the whole car.
So, my advise is: Get help from a more experienced friend ASAP, or find yourself a garage that is willing to have you look over the shoulder of the mechanic and ask the hell out of him.
Also, get yourself a Haynes manual, as reference. It is by no means a real instruction on how to (dis)assemble your car successfully, but it is better than going blindly.
If possible, get yourself english or portuguese garage manuals from Volvo, they are bloody expensive but they contain everything you would ever want to know and pretty invaluable IMHO.
As for tinkering with the LPG system: Don't even think about it as long as you are not completely intimate with the car AND the system! First make the car run OK, then read every bit of information there is on both engine and LPG system, and then, maybe, if there is no experienced mechanic around to do it FOR you, maybe you can try it yourself.
LPG is dangerous, it is pressurized, don't mess with it.
Now go and find yourself some guy that can pop the hood with you and check everything.
