Yes they do make an 18 and yes Nigel Foster designed it.
Be careful when buying a used Point 65 since initially they had quality problems (I believe they were built in Romania or somewhere similar before switching to China) and the older ones are still floating around (pun intended). Anyway, I had a X-Lite that I purchased new at a used price - go figure - that had a few problems, but after I fixed the leaks and such it was a great little boat that tracked like a dream, but was somewhat of a tank (55+ in fiberglass at 15'5). The rudder was removed (never worked) and the skeg cable soon detached from multiple places from the hull rendering the skeg useless. As luck would have it nether were needed since the hull has such a deep V and the chine so hard. I recently sold the boat, but my plan was to have Sterling Fiberglass remove the skeg and skeg box making room in the stern for more gear. Also, at least in the X-Lite's case the boat was VERY tippy if you don't have excellent balance starting out make sure you have a good roll and a dry suit before venturing out of warmer waters. With that said, I actually miss the little guy and its comfortable cockpit, (oh yea, the back band support snapped too), large hatches and layup that would make any Brit proud. Hope this helps.