Hi All..I'm new here..It might seem a stupid quesion to most of you..Bit you never know,if you dont ask..lol..

That's exactly right Sailshack.
Greg, you probably want a width of about 30 inches wide for starting out. Sailboards have only recently hit that width.
The big old One Design Boards probably slide along OK, for flat water touring. I haven't tried.

So do you want it for surf, or flat water touring?
Have you been in touch with you local SUP lesson provider, for a try?
There are some cheapish 2nd hand boards coming onto the market too.

Not a stupid question Greg, after trying sup a few years back at a windsurf comp I was instantly hooked. what a predicament, I couldn't just go and buy a board on a whim so I bought a paddle instead. Already had a board, it was my slalom windsurfer 55cm wide and 100 ltr volume, quite challenging to say the least, but when I could afford my sup it was sooo easy to paddle.
worth a try, why not?

Pacifico is stable at 30.7 inches wide and sails pretty well (see photos at bottom of attached link).

www.kitforsale.com/news-189.html

I think some old sailboard make great flat water SUP. I paddle old windsurfer one designs on the river a lot and for their length they glide very well, flat rocker and a slight displacement with narrow width helps a lot. I also think the weight helps to maintain momentum on flat water.

I also paddle an old div 2 type sailboard (crit d2,round bottom) and it flys for its length (around 11 ft). Its quite unstable but like anything get more stable with speed.

These round bottom boards can generate speed by just rocking them with a bit of centreboard down. (illegal in sailboat racing). I wonder if the SUP racing guys have looked at this technique while paddling to generate more speed.

If all your looking for is a board to cruse around on flat water an old one design does the job well and they can usually be found cheap

nev