This is something only you can answer. If you are not bothered about the extra performance then i would recommend buying as new a MCS you can find with the right spec for you.
The MCS JCW is faster than the MCS and in the 2nd hand market they only tend to command around no more than a £500 premium. Some people are put off by the higher insurance. So in my opinion, assuming insurance is not an issue, it is worth the extra money.
For 10K you can stick to the 54 reg onwards car, more likely 06 onwards, which is a good thing as these are the updated R53 cars.
The MCS JCW is not really any different to the MCS in terms of issues. Most are sound and if they have been serviced and less than 70K miles you shouldn't see any major problems with them.
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