Hi Sammy,,,
The Marauder is definitely dependable enough to buy. I have one in .22 caliber and I can tell you they are great rifles. Mine is powerful, accurate and so quiet it's amazing...
As for your other questions.................
#1) You should sight in your pcp for the power level that you intend to use on a regular basis whether it be for target shooting or hunting. Changing the power setting of your rifle can indeed change the POI of your pellet but it's easy enough to resight in the rifle for the new settings,,,,, or you can get a good mildot scope and use the mildots to compensate for any POI change that might occur.
#2) Personally I don't think an RWS is that loud. I have one and I shoot it all the time in my backyard range and have never had a problem with it being to loud for suburban use. As for the sound levels of pcp's,,,, it depends on the specific rifle. My Marauder is mouse fart quiet however my Discovery in it's "stock" form is way to loud for me to shoot in my backyard,,,, same goes for my Air Force Talon and Condor.
#3) Yes they are :)
Remember though,,,, if you go PCP you'll have to obtain the necessary equipment to air your rifle up. You can go with a pump, a scuba setup or spend a large amount of money and get yourself a compressor.......
I'm sure other members will chime in with their advice.
Jeff