I have a Sea Tour 15, and it is indeed a stable kayak, once you get in it. I have seen people who are used to bigger boats with a flatter bottom find entering it tricky, so it may feel a bit tippy until you get in and get settled. We're the same size so you should be OK there. Given the dimensions of these boats are more like commercial sea kayaks than traditional Greenland boats, you don't need a traditional paddling instructor/club to help you out unless you intend to use a Greenland paddle. Regular sea kayak lessons are fine.

These boats, because they lack bulkheads, do not lend themselves to re-entering the boat if you capsize and can't roll. Even with float bags mine submerges completely if I try to use a paddle float to get back in. Rescues with partners work fine, but solo ones are not easy at all. I paddle alone pretty often, and if I was not extremely confident in my rolling I would not keep this boat. Solo paddling + No roll = no SOF boat, in my opinion.

Unless you are going to be paddling exclusively with other more experienced paddlers, I would buy a used plastic sea kayak for your first season while you build the SOF, just to get your feet wet. It will make the learning curve much easier and you can sell it for what you bought it for when you're done (I see used Prijons for ~$500 all the time)