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1) Since I want max time on the water, eg low wind conditions, should I go for a formula, slalom, or a more recreational board.
Formula is great for course racing - they go upwind and deep off the wind very well but they do not go across the wind as easily as a large freeride board. Slalom is fast but you give up a certain amount of comfort. You can still go plenty fast enough on freeride gear. Some freeride gear will be more speed oriented (sometimes called freerace) and others more manouvere oriented. Make your choice based on comfort I reckon - the faster the board generally the flatter the bottom and harder on the knees.
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2) what size board for my weight?
I weight about the same as you. If you are looking at two boards eventually, you might want to start with one about 75-80 wide (130-140L usually). This can become your lightwind board when you get more competent and go for a 60-65 wide board (100L) for some real fun.
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3) How many and what size sails for max time on water?
As Moby says, three sails will cover a wide wind range, particularly if they are late models. You need to consider boom and masts. You probably need to end up with a 430cm and 460cm masts. For light wind a 7.8 is as big as you can get for a 460 mast. A 430 mast will handle low 5s. A boom that is about 170-220 may fit all three sails.
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4) If I can get a second hand board what is a rough estimate of cost to get on the water.
I have bought boards from new $2K+ to as little as $250. Sweet spot I reckon is two/three seasons old around $600-$800.
Something like this for $500
www.seabreeze.com.au/Classifieds/Windsurfing/Boards/~bahrj/2006-Fanatic-Shark-142-Ltd-264-cm-142-litres.aspx?search=vV34ma7TLlfSOOW509XZB%2bDPcbmf%2bVQPXVyzDxYhRvFHcDAH7eNi6Q%3d%3dAll the manufacturers have something in this space.
I agree with Moby about starting. At our local place you can get a bunch of lessons for pretty much the price of hiring the equipment - the boards are really stable and the sails are really small, you'll get going straight away. Once you've got that sussed, get your big board and smallest sail. As you get more proficient add more gear.
I took it up four years ago and love it. I still remember the first time I got both feet in the straps, fully planing. Wooohooo. You're in for some fun.