A good aftermarket HU will be a good starting point to a decent sound system.
Replace the door speakers with improved units, and add an amp to drive a sub, and that will sound better than the factory HK offering.
The HK amp needed the wiring to go with it, which was incorporated into a special harness.
An aftermarket amp will connect to an aftermarket HU very easily, much easier than trying to make an HK amp fit into a vehicle that wasn't wired for it.
The HK amp really isn't worth the trouble, if it's not already fitted. Outputs wise it's rated at 6 X 25 Watts. The door speakers received 25 Watts each, and the boot mounted sub got 25 Watts to each of it's voice coils, which totalled 50 Watts to the sub, giving a total of 150 Watts for the system.
The system I put in my FL1 used a Sony HU rated at 4 X 45 Watts which powered the door speakers directly, and a 300 Watt amp powering a sub in the boot, which took the sub signal from the HU.
The factory door speakers were "OK", but I upgraded them to JVC coaxial speakers, which were a big improvement in volume and clarity of sound.