Career -Vet

Q1, Subject: I am old school, so I will just assume that when you say S.3, that's same as Form 3. Obviously, you should take Science in S.4. You need to study Biology and Chemistry.
Q2, Difficulty: In the U.S., vet school's entrance requirement is equivalent to medical school, so you don't have to be the top student, but you have to be exceptional, probably the top 10% of your class.
Q3, Country: In your case, you probably should consider UK over US. I am not totally familiar with UK, but I've been in the US for some time and I also have a relative from HK who's now studying high school in the US and considering being a vet later. In the US, you have to first finish the 4-year bachelor degree first (science, bio, pre-med, etc), then apply for vet school, spend another 4 years afterward, including residency. So we are talking about 8 years of post-secondary study. The cost for the whole package is about US$250,000, and that's the low-end of the range. Of course, if you are lucky, you may be able to get grant, loan, or even scholarship. However, a lot of medical schools in the US will only accept students with US residency or citizenship, and you have to be a US resident or citizen to practice medical (and medical residency as well) in the US. Vet schools follow similar practice, but may have more flexibility. For UK, once you enter college, you take Vet major directly, and we are talking about 4~5 years time frame.
Q4, Career: You probably would like to be a vet after spending so much time at vet school, right? Your job opportunity is narrowed, but fortunately, you don't have too much competition as there are not that many vet schools around, and not that many vet graduates.
Q5, English: You don't have to be a native speaker, but you at least have to be able to communicate. Broken English is fine as long as you get the information delivered.