I'm not convinced about a 4 door convertible like this, although as Chris says the roof doesn't really add much to the overall strength, that's why if you look at the crash-test photo's the windscreen pillars and roof fold up first.
One of the benefits of welding up the rear door area is that it helps to strengthen the B/C post, from experience I know they flex quite a bit when not attached to the roof, so I bet they really flex on this when you open the rear doors. At one time I considered doing a 4 door convertible, but I was going to convert the rear doors to suicide format, hence moving those loads to the D post. Still, the side-impact protection aspect of the B/C is severely limited.
I assume there is no hood, can't see how anybody would buy it with no hood, and certainly not at that price, even with the original roof in place it probably isn't worth that, and removing the roof without any additional work certainly doesn't add to the value.