My vote is for the horses and wagon. I can't nearly make my share of the payment (I can only make a mere 43 gp contribution.) though I'm fine with owing some money to the others.
Concerning using a bag of holding, Assuming the water per day weighs the same as rations per day (wich is far on the light side), that is 480lbs of rations and water for the group per month, which would require at least a Type II bag, which costs 5,000 gp, and that's just for one month. The bag is right out.
If we come up short on money, we can switch to donkeys, which will increase the travel time by %33, but decrease the cost by 384 gp. Remember though we would likely be able to sell the horses at the end of the trip.