For start maybe a North sails duke 6.4 or something in that range? Gaastra IQ, NP firefly....

unless money is no issue i'd definitely go second-hand as the chances of you giving it a good smashing in the first season is pretty high. as a beginner, you're going to frequently end up falling hook-first into the sail, as well as endure the odd catapult...your sail will age very quickly!

size-wise, i'm with racer-x...you'll need a reasonably large sail to get planing in good learner-intermediate conditions (20knots on a 180lt board will be lively to say the least!), so look into the 7metre range. i've had a look in the buy-and-sell on sea-breeze, and can't find anything in NSW for you but if you look further you might consider something like these
(note - i haven't sailed any of these, nor know their owners, just giving you ideas is all)...

www.seabreeze.com.au/Classifieds/Windsurfing/Sails/~bl0eu/2008-North-Ram-F8-68-metre.aspx?search=P6K%2ftsb2rZrARE%2bQ6m0Zxw%3d%3d
(appears to be camless and in good condition)

www.seabreeze.com.au/Classifieds/Windsurfing/Sails/~ba0gu/2000-Neil-Pryde-Supersonic-74-metre.aspx?search=P6K%2ftsb2rZrARE%2bQ6m0Zxw%3d%3d
(well looked after for 2000! can't quite make out the specs, you'd need to talk to the owner/look it up on the web beforehand)

matching mast ICMS with sail requirements will make the sail perform much better - do your homework and ask on the forum if you're unsure.

good luck

hello:
i also am 100 kilos
i started with old 7.0 and epoxy mast

once in harness and learning to "fly" , i purchased a new 8.5 no-cam sail with appropriate 490 mast
that sail planes earlier for our weight and has a HUGE range 12-20 knots
if steady 20 need to go back to the 7.0

the 7.0 was only good for about one year, but i had done long-boarding before:-)
now i even use a 10.0 cammed sail with 520 mast to go out in 10-15 knots :-)
{after 2 years}

where we sail there is only small chop ie NO waves - so, we can go bigger !!!

enjoy !!!

Thanks for all your tips!!!

At the moment I have an NP Exess 6.9 on a 4.6m mast and some times I get it planing with hooked in harnes lines but mostly only for a short time (board turns into wind - prob. to much backfoot pressure, wind drops off or I run out of water - should go further out)

And yes, the board gets very lively when planing ...

Thanks again for all your tips!

I have to talk to my "financial adviser" now ...

Try moving the mast foot forward a bit and harness lines a bit to the back, only few cms will change the behaviour of your board a lot...
6.9 excess is a good sail, I liked it a lot when I used it...keep this one and go for 7.8 or 8.0 to add for lower winds

...
100 kilos - "flat" water - go for 8.5 next :-)

I have experienced something similar as you said, now try variations, of moving the foot by 1cm or 2cm to find what suits you best.