I would advise you to contact forum member chop5 if you are looking for a PC Engine converter. He makes homemade converters using ribbon from other electrical equipment. They work really well and his asking prices are reasonable.
Getting a japanese system card is definitely the way to go. I would advise getting a PC Engine Arcade Pro card at some point which would allow you to play every single PC engine CD game and you would never need to buy another system card. Make sure it's an arcade pro card and not a duo card since you are using the CD attachment. Unfortunately at the moment, the prices of arcade cards have been on the rise, but nothing compared to what US system 3.0 cards go for.