Well, the fact of the matter is that save games will not be compatible in the least. Its possible in theory to build a save state converter, but it?d be a lot of work and only a few select people would benefit from it. I think most people will want to start from the beginning anyway and fully experience Tsukihime in English.
Any version of Tsukihime should be compatible, so long as the arc.sar is present and is valid. Really, that is the only file you need to play our patch, as ONscripter replaces nscripter. If you want music you?ll need the CD though, although you can rip the music to mp3/ogg (can?t remember which it is atm) and put it into the game directory as well.