Hey, I'm a BME freshmen and I was accepted ED II last year so I figure I can answer a few of your questions.
First of all, I honestly don't know much about how well ranked ME is at BU compared to other schools, but I do know that BU's engineering program in general is quite respectable. ENG is one of the most selective college's at BU, and has one of the toughest curriculems.
From what they've told us in the freshmen seminars, most engineers go into the field with just a BS, but I think around 20% continue on for a master's degree or further. I've been told that most employers hire engineers with just a BS and then will later pay for them to go back to school to get a master's degree if they decide they like you and want to keep you working for them, and this will probably be my plan after I graduate.
My stats last year were an overall GPA of around 3.4-3.5 with a senior year GPA of 3.7ish. I also took 3 science and 2 math classes senior year, and I think that helped me out. My SATs were 1380 with a 680 in math and a 700 in CR. My writting was a 630.
The other schools I applied to were WPI, Tufts, NEU, and Johns Hopkins (super reach). Comperable schools to BU in terms of engineering would be WPI (a little easier to get into than BU) and Tufts (a little harder to get into than BU).
Personally, I'm really happy with where I ended up, and I like the ENG school a lot. If you do choose to go here, be prepared to do more work than just about every other major. Any engineering is going to give you a ton of class hours (I have 24/week, mechanical engineers have around 21, and something like a psych or IR major will have around 14-15). I'm not saying those majors are easier, I'm just saying that ENG has tons and tons of class hours, so be prepared for it.