Modding your PS2 to play imported games voids your warrenty, so if your PS2 ever breaks you can't get a new one and you have to buy an entirely new PS2. KH2: Final Mix has no English subtitle settings, because it was intended to be a Japan-Only game, so other than spoken dialogue in a few cutscenes the game is completely in the Japanese language (all the cutscenes with English voices were already in KH2, any newly added cutscenes in the game are completely Japanese)
Buying KH2: Final Mix+ is not worth it. Every extra tiny bit of content that it in the game has been recorded and put into videos on Youtube, you'll know everything that happened in the game just by watching Youtube videos, and it's not worth paying over $100 for a single game that's just a 'special edition' of a game you've already played