Maybe then also find a discount bus ticket from Buffalo to NYC? When are you going? If it's during a popular warm-weather vacation time, the cheap seats, if they exist, could sell out even by now.

Hertz has minivans one-way for $132/day plus tax. Again, only you can decide between value and total cost.

If you were a smaller group, I'd recommend trying to earn a free day, which yields a full size car with 5 passenger capacity. In your situation, you'd need to have two drivers each try to earn a free day, so you can have 2 fullsize cars. This option is not easy.