David, I do not think the 31 ever had a soft top. Like David said, the 31 is basically the same boat as the 30 except they did some pretty serious redesigning of the interior spaces and cockpit space. They basically moved the galley from down below up to the outside cockpit area and also redesigned the seating. I think the redesign was a huge improvement for the boat. Assuming the hull is the same as the Pilot 30 II, and I think it is, it is a fairly seaworthy shape of a hull. I had the one in my signature for 6 years and loved the boat.
Build Quality is average. Nothing earth shattering. I will say that build quality of my Carver is surprisingly better than the Mainship. But that really doesn't mean much. For what you say you will use it for, I think it would serve very well. 4 people would be tight but you mentioned it with the "odd weekend" meaning you won't be doing that much. And if you are European you folks tolerate smaller spaces and closer living quarters better than us Americans.
The reason you don't see much about that boat is because they started building the 31 in 2007 and Mainship went bankrupt in 2008. SO there were very few 31s built. They do command a fairly hefty price though probably because they are newer and the redesign really was extremely valuable...in my opinion.
Prairie 29...Perkins 4236...Sold
Mainship Pilot 30...Yanmar 4LHA-STP...Sold
Carver 356...T-Cummins 330B