I fear your question is beyond the experience of anyone who would be responding here. As much as I think I'd enjoy a Catheram, I've never driven any of them. If speed isn't a priority, then the Roadsport 125 should be plenty of car. Otherwise, the 175. I do see that they offer both the standard and SV chassis, with the latter offering a little more room for people and luggage.
The larger question: any interest in assembling it yourself?