VV, welcome, and I'll add a bit on some of your questions. 2. 3., don't know what's meant by an 8.1 system; 7.1( left, right and center in front, left and right side surround, and left and right back surround, plus a sub)are the usual full setup. In your price range two models that come to mind are the Denon 3805 and Yamaha 2500. Same for the M60s.
4., 5., don't follow what all the speaker points are intended for. You shouldn't have different speakers in different positions handling the same channel and possibly muddling the sound.
6., competently designed receivers, certainly including the ones which you'd be considering, have all the voltage regulation and power conditioning necessary done in the transformer, rectifier and filter capacitors in their power supply section; no further treatment is necessary.
8., side surrounds(when there are also back surrounds)are best placed even with or slightly ahead of the listening position.
9., 4 ohm speakers(there are almost no 2 ohm home speakers)have a lower impedance to current flow than an 8 ohm speaker and would draw more current if voltage was the same(Ohm's Law; I= V/R, or current in amps equals volts divided by resistance or impedance in ohms). As long as the receiver/amplifier is able to supply the increased current without shutting down as a protective measure, 4 ohm speakers are fine. The receivers mentioned above, among others, can handle this.
