You really don't want a Kohler toilet!
Is the existing one pressure assisted? Those are always somewhat noisey. A good gravity fed toilet is typically quieter, and may not run anywhere as long if your existing one is very old (and therefore uses more water which has to be replaced on each flush).
A favorite here is the Toto Drake. It's available as the Eco-Drake version which only uses 1.28g of water per flush. There are others that work as well...somewhat depends on the style you are looking for.