Word of advice: Do NOT let "proffesionals" do your installs.
They dont install your gear into your car, they hack and mash your car to fit the gear.
Go and watch a few places on the wkends doing installs. They dont care about our car. They dont care about the audio gear either. They just pump the installs out and take your money.
Speakers and head units are the easiest shit around to install.
speakers are usually unscrew, unplug, plug, screw, done.
Head units require a little more work but the hardest part in the cars ive done is figuring out which wires go to which speakers on the unit your removing. Multimeter.
Just do a bit of research on the net, read the manuals and have fun with it.
Make sure you put aside an entire wkend and you have plenty of mates and beer to help. Shit will ALWAYS go wrong so youll need time to be creative before it gets dark
I have more fun putting in stereos than i do cruising around because you have a sense of accomplishment afterwards.