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I love my iMac and have not had any of the problems you describe. I love it and it always works great I have never had a wifi problem or ever had it make any kind of loud noise that you describe. Personally I would purchase an iMac above a Mac mini its more powerful and nice and sleek.
 
I have a late 2009 27" iMac.. i7.. 18 gigs of RAM.. basically maxed out.. and I have not had one single issue. The ONLY issue.. and I literally mean the ONLY issue I have with this computer is that the USB's are on the back, so I had to get a powered USB hub. Other than that, this computer works the same as the day I got it from UPS. Never once locked up, and literally no issues whatsoever. I will use this for my primary computer for years to come. There is no sign of this thing slowing down.
 
I like my 2010 i5 27" model, the only thing i would change is having an SSD inside of it, i'll probably do it once the extended apple care runs out, maybe prices have dropped enough that i can actually replace the HD outright (256GB+) or do a stripped of 2 ssds removing the optical drive and keeping the HD for fast backup.
 
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