How many hours per week would be reasonable to work if I am studying full time a fairly demanding degree (3 days at uni plus some online lectures)?
Depends how demanding the degree is outside of your time on campus I guess, and how much money you need. Online lectures aren't a big deal if they are recorded, then you can watch them whenever you want, but if they're live then that will be inconvenient. If you don't need to do a huge amount of work outside of your on-campus time, then work more. If you have a very demanding degree in terms of self-directed study and time-consuming assessments, work less during semester and more during breaks.
I still don't know much about GPA/WAM
Simplified: WAM (Weighted Average Mark) is the average of your marks, GPA (Grade Point Average) is the average of your grades as allocated a numerical value. Different universities use one or the other, so find out which your uni will be using.
Generally (majority of universities, so clarify for your uni, but I'll use this for my example):
Pass: 50 - 64 (4)
Credit: 65 - 74 (5)
Distinction: 75 - 84 (6)
High Distinction: 85 - 100 (7)
So if you scored 68, 75, 83, 86 for your semester one subjects, and 75, 80, 84, 88 for your semester two subjects, your WAM would be 79.88 (if your uni is nice, this would be rounded up), and your GPA would be 6.125.
It has the potential to get more complicated at some universities in some degrees, where they introduce weighting to later years, or use a degree WAM and a major WAM. For most people, in most professions, WAM/GPA are not particularly important unless you want to do postgrad where you will be competing and some courses have minimum entry requirements.