There are some things all graduate programs in philosophy in the US/UK are looking for:
- Strong analytic skills
- Basic familiarity with logic
- Good writing in English
- Good grades in classes (especially classes that show reasoning skills)
- Strong writing sample (demonstrates 1 and 2)
Where things will differ,
For history oriented programs, you're definitely going to want to demonstrate strong knowledge not just of the era you imagine writing your dissertation about but more broadly. Here, it would also help to have relevant language skills (so if you're talking University of Hawai'i -- Chinese or Japanese or Hindi but if you're talking Emory, then German).
For analytic programs, a much stronger background in logic will be expected along with familiarity with contemporary metaphysics, epistemology, and philosophy of language. Probably, some coursework in philosophy of mind would also be good.
Turning to your questions,
Since you're saying history of ideas, it would seem to be a pretty bad gap to lack coursework in Kant.
For the writing sample, it could negatively affect you in two ways. First, undergraduate students usually don't know how to write a good philosophy paper, and part of the point of coursework is to practice and get feedback. The more classes where you've tried writing such papers the better. This is primarily a problem about form and writing methods.
Second, lack of knowledge of Kant would negatively affect the content of papers in quite a few domains. Without having read Kant and understood it, it would be hard to interface with determinism/free will, philosophy of mind, contemporary ethics, phenomenology, Hegel, or epistemology to name a few domains.
Now if you're getting your writing sample read by someone competent, they might be able to steer you away from quagmires on that front, but these are just my two cents (or eurocents?)