Roy,
You can use either 27 or 40 Mhz, but you would be better with 40, or even 2.4 Ghz. 40 has more frequency options than 27, 2.4 has unlimited frequency options.
As for the number of channels, it depebnds on what you want your boats to do. It might be worth buying something with multiple channels so that if (when) you get hooked and build something bigger and more complicated you have the channels to work it.
Ian