If you are thinking that your apiary may grow in the future then think a lot bigger than you think you need. The cost is usually not proportional.
Also for the ultimate disease break, if your extractor takes a super of frames, 8 frame boxes, 4, 8, 16, 24 etc, so your frames go back into the same super. A small BK, 60 hives has his hives and supers numbered, 8 frames, extractor 16 frames, and the supers with their frames, go back to their hive after each extraction.