Any SUP with a mast base, but the ones that just have one brass insert can be weak - make sure the thread is cleaned of salt and sand, else you can spin the insert in the board real easy. In that sense, getting an old SUP and fitting a mast track is probably better (and not costly)
For sails, go to the WA part of the windsurfing forums and look at the free stuff. You will want about a 4m to 5m wave sail and will find such easily enough. Probably a boom and base too. If you get a free mast base, change the rubber part in the joint. Masts have to match the sail so might cost you a few bucks but you should be able to put together a full rig for $300 or so
As an example, the first sail here (blue Arrows Impact, gone now, but would be ideal):
www.seabreeze.com.au/forums/Windsurfing/Western-Australia/The-FREE-stuff-thread?page=26