I would picture the MSc programme as an intermediate goal, and assume you will be admitted and obtain it; in the letter, explain how you're going to use it to make the world a better place.
Also, and more importantly, show you know their programme in depth. Say which professors you are looking forward to studying under, and which aspects of the campus, the programme, the university have attracted you. Show you've done your homework on them.
That would be very confident and credible. What you've got is basically the same as other student letters.