The frequencies you mentioned are IF frequencies. The C band range is 3700 Mhz to 4200 MHz. = 500 MHz.
So all these LNBs bring down signals from that frequency to 1GHz range so that it can be carried in a coaxial cable. What matters is the Noise figure.
Same forum has a post on 12K/17K LNBs if you are concerned about the 12K/17K. otherwise, you shouldnt be worried.
if you are targetting Thaicom, they do have transponders from freq 3400Mhz [it is a old satellite and the world decided not to use upto 3699Mhz to avoid overlaps]. In that case you need to choose a old LNB that has 3400 -4200Mhz Input range, Forget o/p range, all will abide to rules.
