I think you'd do fine in physics 1B03 since it is similar to high school physics. It would probably be harder than 1L03 but since you already took gr.12 high school physics I really don't see the point in wasting your time with 1L03. (But it's up to you) Keep in mind that some second year specializations require you to take physics 1B03 anyway so might as well get it out of the way in first year.
" PHYSICS 1B03 must be completed by the end of Level II. PHYSICS 1L03 serves as the prerequisite for PHYSICS 1B03 for students who have not completed Grade 12 Physics U. Completion of PHYSICS 1BB3 is also strongly recommended. "
^ I got this from the course calendar for biochem. I don't know which specializations this applies to.
In regards to microecon vs. enviro, I took microecon and it was beyond easy. I had taken a similar course in high school which definately gave me an advantage but either way, I recommend this as a grade booster. I didn't buy the textbook, never went to lectures, only took 5 quizzes, and walked away with a 12. The midterms were very easy and I thought the exam was pretty hard in comparison but still 12-able.
Sorry, I can't help you with the bio 1a03/1m03 thing. But I don't think it matters which one you take first.