Generally the entry requirements for BEng are made slightly lower than MEng, but that shouldn't really be a factor in your decision because you will know what you are capable of acheiving and most MEng courses ask for the same entry requirements as other undergraduate courses.
As for continuing BEng to MEng, this is definitely possible after you have done BEng because the first 2 years of the course will be the same, so you don't have to do it from the beginning. For me the decision was based on finance. I knew I was capable of getting the slightly higher grades to start MEng from the start (which really isn't much higher, if different at all) and it works out more financially if you start with BEng. It really doesn't matter too much what you start with, I just think personally that you will benefit more overall by starting with MEng rather than BEng. Also just to add, you will pretty much have to do MEng because most careers in Engineering require that you have an MEng.