I don’t think the size of the lake matters much. The 18 goes maybe a few mph faster than the 16 at best, hardly a big difference when it comes to the room you need to sail it. The 18 is just a smoother and more forgiving ride, but also has more adjustability now the rig and can be tuned to go faster. It also handles waves and more crew weight much better.
An SX rig will be tough to find. There are some out there, but they’re a bit more on the rare side. Similar to (but not quite as rare as) the Hobie 20 and 21 variants. The SX has a longer mast, taller sails, longer shrouds/forestay, and the SX wings, which were also sold as an add on. SX wings are the easiest to come by, as many people bought them as extras. The mast will probably be the hardest. The sails will all have to be custom, as the originals were mylar and are certainly all ruined by now. The shrouds and forestay are still available or easy to make otherwise. There are a few people on here that have SX rigs and love them though, John Linn being the one that immediately comes to mind. He also has a custom spinnaker setup on his. I don’t think you’ll get him to part with it though!