Hello Barry
Not sure that I am the best person to answer you as I have only had Blue Moon (325/32) for 4 years but here is some of my experiences.
The 31 and 325/35 both have no internal ballast , the only ballast they have is their lead wings on the end of the keel, this gives them a very good ballast ratio and certainly makes them very stiff in a breeze, the 325 more so than the 31. The Super Seals and 27's rely on internal ballast which makes them far less stiff and I notice that they have to reef in a lot less wind than we would. As for sea keeping, I have to confess that I have not sailed in many other boats to be able to make a comparison but we know that the seas around West Wales are not the kindest and Blue Moon is quite happy there. Also note John Margarsson sailed our 31 to the Azores two years ago, OK the mast fell down one day off the Azores but he got there safe and sound and well ahead of all the other Jester Challengers.
Engines were generally the Yanmar 2GM20 which always seems to get us out of trouble but sometimes I think I might be happier with a little more power and a bigger engine is on my wish list(Not top).
A lot of the 325/335 owners seems very happy with their boats hence lack of boats on the market however remember a lot of owners are not members of the Parkerseal assn. and so boats do come up for sail through brokers and word of mouth, certainly Pascal (Maruli 325/27) found several to choose from when he made his searches.
Try and get a trip on one and see if it is right for you.
Jan Newman
325/32
Blue Moon