Hi iLog,
Don't do it mate, the 5'10" WAM is way too small for 100kg. Can you ride it? Yes, of course. Will it help you to learn to ride waves? No, the tail is too narrow for stability and light wind planing. It will be a nightmare for you to learn to gybe and will bog you down with spray off the rail hitting your back foot.
Last season I was 96 kg (lost a few kg since then

) and rode the WAM 5'10" on a downwinder in reasonably powered conditions on a 9m Reo. I didn't like it at all. The tail doesn't have enough surface area to support the weight so it will be constantly bogging down on the wave after you have washed off half your speed in the bottom turn. This board is perfect for 75-85kg riders IMHO. North recommend the 5'10" WAM for riders between 70-90 kilos.
You could try the 6' WAM but if you are looking for light wind, small-medium waves, then look to a 5'3" Nugget. That will work perfectly. You could also look at a Whip in the 5'5" size as it's wide in the tail and will give you the lift you need. The Nugget is stable, planes early, goes upwind well and will add to your days on the water. They work best in 12-20 knots depending upon kite size. If you have a Juice with a Nugget, you can go in 10 knots easily.
The price is only so so, if it's a standard construction. They are $999 brand new and you could pick one up on end of season sale for around $750-799 which is not a massive stretch from the $610 AUD you are talking about.
If you want to ride in strong winds and powered up, the WAM 5'10" will be OK, but from what you've described, I'd keep looking.
DM