Which amplifier should choose for such setup? 2 way or 4 way (2x channel plus one bridged for sub, which is already an active one)? Is there like any specific spec that I should look for apart from RMS power per channel?
Well, if you go for an active subwoofer, you will need an amplifier to drive just the front speakers and with a pre-amplified output to carry the signal to the subwoofer (if you want to avoid add an extra hi-lo signal converter).
Now, if you are going to install those Gladen SQX 165s, which have a 2 ohm impedance, you will need a 2 channel amplifier with 2 ohm drives compatibility (aka 4 ohm x 2 channel bridgeable), with at least 30 W RMS for start to moving them, and up to 140 W for a relaxed amp work (this at 2 ohm specified power. 15/70 W RMS at 4 ohm if bridge capable).
For last, in the car audio world you need put attention over other parameters in relation, specially talking about speakers, which are also relevant if not more than the power handling, size and the mentioned impedance. These are sensibility, frequency range and resonance.
I think that the frequency range is easily understandable. A subwoofer can reproduce 20-600 Hz range, a mid-range 500-5000Hz, and the tweeters from 1000 Hz up to 100 KHz (some special high fidelity ones). Out of there, them will start to distortionate the sound or will not be able to reproduce some frequencies.
Resonance is where the speaker cone moves more and more comfortable with less energy needs, or with other words, the optimized frequency of it. So, if you have a sub with resonance of 80Hz, the High/Low pass filter curves should be adjusted to limit the frequency range in order to center it at the resonance frequency (start at 20Hz and end in 140Hz) to get the optimal sound quality at lower electrical consumption.
Sensibility is the most perceptual parameter of a speaker, and it is the division between the acoustic power radiated by itself and the electric consumption. In other words, how much amp power is needed to move the speaker, so more sensibility - less power, with more power and sensibility - more acoustic volume sensation. But more sensibility can carry more or sooner distortion, narrower frequency range or linearity lack (some frequency ranges can be heard more than others).
As example of this, a 35 W /100 dB speaker can produce same acoustic sensation as a 120 W / 90 dB one, or a 120 dB super tweeter/trumpet will be heard more than a SPL subwoofer, but it will distort sound easily, and much more if the frequencies are out of its range (500Hz for example).