Of the four boats you mentioned, I think the Hobie 18 is by far the best choice, based upon how you said you plan to use it. It is a great boat for couples because it has the roller-furling jib so you can shorten sail if the wind comes up. It is a great singlehander and performs very well with main only. It has enough buoyancy to carry up to four adults (or lots of cargo). It is fast but forgiving. And because it is relatively heavy compared to the more modern designs, it is also more stable on the water (not as quick to fly a hull or to capsize), which I would think would be important in your cold water conditions. My husband and I raced a Hobie 18 for almost 15 years, pushing it hard, and we never pitchpoled it. It's very adaptable to a spinnaker or a roller-furling reacher (Screacher or Hooter). I am a 130-pound woman and I have no problem singlehanding a Hobie 18. If you are not accustomed to a boat with daggerboards, you will quickly learn to deal with them.

HOWEVER, if you are planning to race, you have a whole different set of criteria to consider.

And no matter what boat you get, if you are singlehanding most of the time, you should be sure to have a good system for righting the boat, whether it is shroud extenders and/or a righting pole. For singlehanders I also recommend some sort of mast flotation device so the boat cannot turtle.