I did exactly what you are asking, I bought a junk black PC Engine Duo, here's my take on it.
I bought a Duo that was listed as HuCard/CD working but no sound at all. The easiest part of the repair (for me at least) was installing the cap kit. There was a partial cap kit already present using leaded caps, I replaced them all with proper SMD caps. The tough part was repairing corrosion damage and in the end, one bad resistor in the power section for the audio amps. The PC Engine Duo, when its caps expire, tends to get corrosion damage from all the leaky caps. If you don't have the patience and detective skills to repair corrosion damage and single out bad passive components, then save up for a recapped Duo.
The worst part is all that was with a really good junk Duo, all that was wrong was missing sound. Imagine working on a Duo where the HuCard slot doesn't work, or the console doesn't turn on. The most common failure is the CD drive doesn't work, which is an entirely different can of worms compared to just repairing the mainboard.