If you are doing it yourself Jaycar is as good a place as any to start. Especially for hardware. You don’t pay for the “name says he who has an Alpine/JL system. Lol. Seriously though for a tight budget and trying to get a bit better bang for your buck they are worth a try.
Beyond that there is just so much out there at a pretty good price these days you won’t go much wrong. Have a listen and make sure the head unit does al you want. If you are looking to build on the system then you probably want to get a head unit with at least two RCA outs so you can add another amp to drive your full range speakers when you get them. And you will, trust me , it’s a slippery path..
The bigger the output your amp has the better mostly for clean power as opposed to actual volume. Bear in mind to actually hear a difference in volume you must achieve an increase of around 3db which equates to almost doubling your output power.
Do not stress too much about your speakers power handling abilities. Most speakers will handle the majority of low to mid priced amp output max power. In fact they are more likely to be damaged by clipping of under powered amps.
Disclaimer – I am not a professional installer or expert. My humble knowledge comes from installing my own and my friends systems over 20 or so years is all. Cheers.