Thank you very much!

What Trail said. If you just want to complete, a plastic sea kayak is fine. You will be doing a lot more work than others on some of the "flat" sections however.
But - like anything else, if you can try some different boats. The favourite multisport boat for average paddlers on the C2C is generally a thing called a total eclipse. Pretty stable but reasonably fast. As a beginner in it you will fall in but it doesnt take long to get used to it. Another benefit of these is they hold their value really well - you could train and race in it for a year and only lose maybe a couple hundred dollars (as long as you don't break the nose off), then buy the sea kayak! The fast end boats are things like evo's, sharps, phantoms, etc. Do not buy one of these unless you are a) a very very accomplished/ experienced paddler or b) aiming at the top 10.