Both of these cars have been discussed on this board quite often. The MBX5 and Kanai 2 are in their own league, and both are excellent buggies. You will pay more for the K2 (about $150) and then you will have to buy some tires. Both are loaded with go-fast-goodies and are considered the best 2 buggies you can buy if you want to race.
The Kyosho comes in several different versions, and you get what you pay for.
Neither the K2 or X5 come with engine, pipe, servos, or radio.
Personally, I'm partial to the X5, but you won't go wrong with either one. Don't worry about the carbon fiber on the X5. It is pretty tough and I've yet to hear of anyone breaking any of it.