Generally in any ICE builds there are 4 tiers build -
1. Subwoofer and amplifier build.
With 2 Channel amp = Bridgemode for Sub
with mono amp = single channel
2. Subwoofer + amplifier + speakers.
with 4 channel amp = 2 Front Speakers + Bridgemode for Sub
With 5 Channel amp = (2F+2R) Speakers + Sub (low power)
3. Custom build. This has combination of mono and channel amps for all speakers and subwoofer.
4. Just component speakers with 4 channel amp
For HU, make sure you get one with 3 RCA pre-out.
For Skoda Fabia my suggestion is to get build no:
4 for improving sound quality by replacing OEM
1 for improving bass with oem
2 for complete stereo upgrade [Check alternator max output]
Check with your local dealer to get idea about speaker size. Subwoofer takes up a lot of space, so make sure you have an idea of it.