I grew up with butterfly's! You must be in the midwest? There used to be large classes
of them there at every sailing club. They are great boats. They are a scow, so will be slightly different in sailing characteristics than a laser or sunfish, but nothing so different to that you will notice learning
to sail. You will have a lot of fun with it!
They are a bit heavy, so I don't know what the limit for a roof rack is, or how able you will be to lift
it up there. I don't know the weight, but it must be around 150lbs.
As for weight, all of the boats in this size will have lower weight limits. They won't sink just because you exceed them. You will learn to sail them just the same. Are you sure you weren't looking at the racing
class weight limit? That would just be the limit of the class and not the physical limit of the boat. On the other hand, if together you exceed 400lbs and are pushing 600lbs, then pretty much no boat this size is going to be very comfortable or easy to maneuver in.
Any boat this size will be somewhat difficult with two grown people on it simply because of space limitations. The boat is basically a surfboard with a foot well. Actually, it will be quite cramped. But the positive is that either of you can take it out yourselves very easily, and you will get used to the space problem when together.