There are a couple ways to go about this mod.
What I like to do is avoid the docking station all together. Just install the RGB amp inside the PC engine itself and then make your scart cable with two RCA cable soldered in for the audio channels so you can plug them into the audio portion of the ifu. Meaning the main cable would plug directly into the PC engine for the video signals, no audio wired up to it, but then two RCA cables go from the end of the scart to the RCA ports on the docking station for audio.
This is a cheaper solution and avoids any possibility of video interference.
The kit that you showed the link for should be a good one though if you specifically want it installed inside the docking station itself.