If you're in WA now give me a bell on 0420998166 - river is running and begging for some fun - can get you a boat no worries. Plenty of stuff to practice in from Walyunga downwards if inexperienced in white water.
For next year see Canoeing WA
www.wa.canoe.org.au/, contact Terry Boland at Canoeing Down Under (site won't load for me at the moment
www.canoeingdownunder.com.au) and join either Ascot Kayak Club (Ascot) or Swan Canoe Club (Mosman Park) for friends, boats to borrow, training partners and tips for the Avon. See also Indian Ocean Paddlers - downwinders rather than Avon, but another club option:
www.iop.canoe.org.au/
Re what class - get out and paddle heaps of boats first and worry about that later. My suggestion is get into a plastic boat first time around. Nothing wrong with fibreglass just that plastic best to learn in - forgiving and cheap in long term. I've paddled most craft downriver, pluses and minuses for all of course.
Kayaks you're in and part of - this hard to beat, however if you come out a fair bit you have a big challenge ahead of you getting your craft back once lost, finding somewhere to get ashore or get craft out of water (I've been stranded in a tree with jammed boat caught in strong rushing water - can't call the RAC / NRMA roadside assistance, baby you're on your own) plus emptying etc. Physically challenging and time consuming - you can get timed-out on both days if you don't meet cut-off times for each section of race. DQ'd and you're out. Skis plastic or otherwise good because easy to remount, fewer dramas.
Anyway I'd get into the water and try lots of boats, you'll soon find your level.