A JD is a essentially a "professional masters" meaning that there are no prerequisite subjects, meaning you can take ANY course. No course will advantage you or disadvantage you AT ALL. With this being the case, it will be to your advantage to attend UoM for undergrad if you plan on going on to the JD. This is because they know the calibre of the incoming students will meet their standards. I would guess that people coming from undergrad at Monash would not be very disadvantaged in admissions but people from some lower ranked universities will be at a disadvantage.
Also, it has been found that hard science majors score just as well in the LSAT as people that study things such as philosophy and politics. So don't worry about that. UoM say on the website that you shouldn't study for the LSAT, but I would recommend you do as (1) Studying often takes people from the 50th percentile to the 90th percentile (2) UoM only offer 50% CSP places, so a higher LSAT could make the difference between paying $24,000 deferred and $90,000.
The JD is 3 years, although it can be accelerated to 2 years. The degree is not much different to an LLB, and most employers would probably rank you on par with someone that received an LLB from UoM and slightly above someone that received an LLB from Monash. But the difference is so small that I doubt it would ever come in to consideration when going for 99% of jobs. University is not very difficult, as long as you are studying something that you want to learn. I learn't this the hard way after taking an accounting subject last semester. UoM, unlike its US counterparts, is not releasing the median GPA's and LSAT's of their incoming classes. Therefore only anecdotal evidence is available. I would say an 80+ average and a 160+ LSAT give you a very good chance.