Hi,
No problem at all. It wasnt too hard at all but as long as you're ok at taking things apart you should be fine.
For the headunit removal I followed this youtube guide:
The hardest part was removing the radio bezel surround as this wouldn't budge and was scared of snapping this but it does take a bit of brute force to a certain degree. Once off then just 4 screws to removal the radio. I taped into the front left/right speakers wires to connect the Kenwood KSC-SW11 sub (under drivers seat) and took the power and switch remote from the fuse box on the passenger side.
I was going to remove the seat to make wiring easily but no need really. The loom from the sub can tuck and run down the side of the centre console to the front then up at the back of the dash behind the radio then through to the passenger side fuse box.
Quite impressed with this sub for the money as didn't want to spend a lot just for some fill in bass which is seems to do well. My days of twin 15" parcel shelf subs, mdf boxes and multiple amps are long gone but depends what you're into, as this can run away once you start.
I would say the Focal ibus range with the right iso adapters is neater solution and easily to connect without cutting wires or soldering.
I would certainly fit the active sub first and see what you think. I feel this will give you the biggest noticeable difference. As said I've replaced the front speakers with 6.5" with adapters from the oem fit which were just over 5" and replaced the tweeters. I cant seem to balance right on these and for the effort I would just start by adding the sub. Reading Allen78 setup, it does sound great though using more premium speakers and been a complete setup all round front/rear/sub.
I will stop waffling now
Sorry in advance, Im not the best at describing "how to" nor is my grammar.
Hope this helps anyway and I have a few strip down photos if required.