The most experienced guys with longboard/sup sailing around here say right in the middle of the board for the mast track. Be sure to have the track inserted into a sizable cartridge of Divynicell or other strong PVC foam. You want it glassed over and routed open, not inserted after construction. Some folks have even gone to the extreme of dropping the first layer of glass into the hole in the cartridge, inserting the mast track, then glassing over with the second layer. Might be unnecessary, but I like the idea.
The Starboard 12'6" has a tuttle fin box for a center fin, and they even made a version with a foot-operated centerboard. I've never found it necessary. You could probably go upwind better in light wind, but who does that? More trouble than they're worth when the wind gets stronger.
For a rig, you need a sail you like the feel of. You don't need ultimate performance. If you don't have old gear, then go used for everything. I love Superfreak sails for SUP surfing. You aren't going to snap-accelerate so you don't need fast-reacting sails. I like the steady, powerful pull of Superfreaks better than the bang, slam of mylar sails. I think it suits SUP sailing much better. The Starboard ultralight rig is a reasonable alternative, but it's built to a price. I think you can get better stuff for the same or less money. I've found used Superfreak sails for as little as a hundred bucks.
Your mast track can double as a carry handle if you have a few finger holes routed into it. Doesn't seem to compromise strength. I route mine to the rear and push the sail forward.
I have four modern windsurf boards that gather dust. When the wind is strong enough for high-performance windsurfing, I'm doing downwinders. The SUP sailing combo is much more fun in the mid-wind conditions. In the ocean, the coolest part is that you're on a surfboard. You drive into a wave and then surf it. I wax my deck pad and then put it in the sun to make the wax penetrate. With the extra grip I can get into a harness and get back to footstrap territory and be fully powered up. You don't get the tail control of footstraps, and it's interesting to jump without straps, but if I can do it, anyone can.