The 85 will not take a TC. Wish it would.
Contrary to Blinking, I have tried to shoot a 300 f2.8 at volleyball, and frankly I don't like it - I would rather have the extra speed and alter my location. Besides not liking shooting at 2.8, and also I don't like the very limited Field of View. If I had my sons video game tuned reflexes, perhaps, but I don't. Lastly, I love shooting a $320 lens, rather then a $3800 lens.
Between the two lenses you mention, I think it is sixes. I have the 85, and can easily recommend it. I have also seen praises for the 100 - and depending on where you will be shooting from - it's reach could be nice. But if you are going to be under the basket - an 85 and 50 would do just fine. I would also look at a wide too just for fun and variety.
I am sure there are others out there that can give you some opinions as well. With enough light, a good 70-200 can work, but again were talking 2x to 4x the price and I don't they they are as sharp as the tiny Canon primes. Cheers and good luck.