Did both Further and Methods in Year 12 having done General Maths back in Year 10 and Methods U1/2 in Year 11. I'll tell you one reason straight up and the only negative I see from this: if you expect to get a high SS, you must get almost every question right and make very few mistakes in the exam. It gets super competitive at the top end and a few mistakes = costly. Aside from this, if you have the aptitude to do maths reasonably well (i.e. that you're doing fine in Methods) and can handle the slightly different orientation of Further, then I don't see any other issues.
Also consider the following three questions:
1. Do you like Further? (I'd assume so seeing as you are considering it.)
2. Are you good at Further? Please see the sample exam from the VCAA website to check.
3. Do you need Further/ Accounting in any courses? I have yet to find a course that requires Accounting as a prerequisite, but Further is actually a prerequisite in some courses (but also covered by Methods).
Anyway, the workload is doable. I really enjoyed Further and Methods. They felt like two different entities though and have two different atmospheres. It's like doing English and Literature/ English Language simultaneously. They're different approaches to the same sort of areas of learning. I actually felt like Further was my 'break' subject back in the day (see my signature to see what else I was doing, excluding Psych which I did in Year 11). I don't think I would've found doing Methods alone would've felt much different to Methods + Further, because, again, they're different.
As for Accounting, never did it, so I can't say. I'll let someone else discuss it for you, but my biased opinion says Further, because it turned out to be one of my favourite subjects. (Also, I help out a fair bit on the Further Question thread, so... yeah...

that may or may not have some impact.)