Hi Matt, I had a 2009 isonic 133 and loved it (really regret selling it) anyway I used to put Neil Pryde 9.2m Slalom sail and it flew, it was a perfect match. At 85Kg I used a 48/50cm fin depending on marginal planning or well powered.
As for the Mistral, bin it, horrible board, it has got to be the worst board I have ever tried, others tried it and felt the same, all it is good for is downwind slalom and then it felt slow and heavy.
If I was you I would get as new as isonic as possible, preferably 2013 models but this is not critical.
Just my two penneth worth!