It depends on what you're applying for. If it's an arts subject, then the writing style is probably fairly important. If it's a science subject then you just need to make sure it's clear and not full of spelling mistakes.
As for content, I suggest that you just write what comes naturally, what particularly interests you about the subject, why you want to study it, what you've done on your own, etc. and then go back through to make sure you've got all the important information in there.
It's generally much better to start with everything and then remove the bits you don't want than to try to write only the best bits to start with.
[b]
