I have Guwange (not to mention other Cave boards), and I can tell you that this will be an adventure. First, it's pretty rare: Most people who have it won't part with it. Period.
Second: expect to pay at least $180 for it. If you find it for that much, consider it cheap. I've seen it go for as much as $250, and even then it's a deal (yes, the game is that good).
Third, make sure it's not a booty when you get it. Cave boards tend to get booted often, so make sure you are buying an original by looking at the ROM chips: they should say Atlus and Cave on them.
Good luck my friend: It is a worthy cause!