To answer some of your questions, having a big amp is certainly not a bad thing. With the tinyest amount of common sense you can keep a big amp from blowing your speakers. When you are comparing power ratings however, remember to look at your ohm ratings. Often times and amp will put out 2 to 2.5 times more more under a mon bridged mode. Basically, an amp that puts out 500 watts in m ono bridged mode may only put out 175 to 225 watts with an 8 ohm stereo load. However, most manufacturers like the highest number of watts available to be the one shown as the "magic" number in advertisements and such.
As far as the fans go, they are not that loud. With a little common sense once again with placement, you can greatly minimize this. The fans will probably come on even under a small load. The fans themselves can also be replaced with an aftermarket quieter one as well. One thing people forget is that heat builds differently in rooms, and is not JUST caused by driving the equipment. If you put it near certain equipment and in a certain type of rack in a certain location in the room, you may actually be driving those fans from external heat sources, and not internal ones.
I am not sure if an amp really does wear itself quicker if it is too overpowered for the speaker system. This may be true, but I can not imagine that we are looking at a very appreciable difference over the life cycle of that amp. For all I know is it may only last 9.5 years instead of 10. That to me would be better than having the wrong amp for 3 years
One advantage of overpowering is both speaker protection, and noise specs. By properly overpowering, you are reducing the chances of your amplifier clipping and taking out speaker drivers. A lot of this depends upon the quality of the amps output, your speakers, and how you choose to use them. If you are constantly driving your system to a point where the amp is constantly hitting pretty hard, that leaves you considerably closer to an accidental square wave or other volukme related instruments. With a big amp though, you do have to pay a little extra attention to your speakers. There are audible signs that you are overdriving your speakers when you are doing it in a slow consistent manor. The real risk with overpowering is that some sort of event (someone floors the wrong fader at the wrong time, feedback etc..) may damage your drivers faster. As far as the noise goes, with a bigger ampo you typically run it further down in its capabilities. With smaller amps they often get run wide open and you get a bit more hiss and such. Of course the better the amp you get, the better it will perform in these endpoint positions especially.
Just some stuff to think about
