You might get away with the cokin holder, but think you might be limited to using 1 filter at a time. Otherwise you may get vignetting in the corners on wide settings.
Do you have any friends etc near by who can let you try the cokin P system out to see if you have any issues with it?
Hitech filters are better than the cokin ones as theres less of a colour cast if you use 2 or more filters together.
If you do get vignetting then the 100mm system would be better as there wont be any vignetting at the wider angles. However that will cost more money