Well, Gambit (sorry, I love that name--ever watch The New Avengers from the 70s??),
There's the rub--it is hard to say how quickly MPI will release further DVD sets after the first one. One would think monthly, but it is unwise to assume anything about MPI's schedule.
I'm sure, however, that if they release the 1966/67 pre Barnabas episodes on DVD, it won't be until AFTER they have finished the rest of the series from Barnabas onward. When they first started issuing the series on VHS, it was not at all clear that they would do the whole thing; they were looking at sales figures, or so I have been told. They may wait to see what the figures are like for sales on the first DVD set before they make a decision about continuing.
Steve