4.5T 3S
But seriously, its something you need to discuss with all the members. You have a wide range of abilities and ages, for a few people running Mardave/Supastox and you would need to cater for all. Thankfully you have a few months racing to discuss and come to a conclusion.
Personally at this time I would go with 21.5T 2S, for both reliability and practical reasons. But that could change over the Winter months.