Well a c/ku horn will work, but it won't get EVERYTHING on Ku, some of the weaker stuff will be harder to get but if the dish is tuned really well and the LNB is tweaked in the scalar it will be just fine for 99% of FTA uses.
The circular is accomplished by putting a teflon slab into the throat of the horn, that allows you to get circular signals. but if you do that you lose linear signals. There's little c-band circular to see from the US unless you have LOS to the eastern side of the arc below 40 degrees west.
If you want to go orthomode feedhorn, it opens up more possibilities, but its much more money. Tell us what you want to receive and we can help you better.