I did UMEP Biology last year, in addition to 5 other subjects (same subjects as you are planning on studying, with German as the LOTE). Although this was manageable, I probably wouldn't advise it. However, as a 5th subject in year 12, UMEP Bio is a great choice! The material taught is not too challenging; some overlap and extension of 3&4 coursework and some new concepts (eg. behaviour of animals, more advanced genetics, detailed plant structure and function, animal structure etc). Assuming that you are doing well in vce biology, you should have no problems getting 5.0 points with practically no study or 5.5 points if you treat it as another vce subject.
Unfortunately, the teacher at my school centre wasn't very good and wasn't particularly helpful. Looking at the assistance and mentoring that I now get at uni, it is probably a lot more comprehensive than what was provided in UMEP bio. I also found the pracs extremely boring :S The travel involved, particularly since it was after school, got a bit annoying as well.
However, if you enjoy biology and are fairly good at it, you should definitely do umep bio to gain a few "easy" points