Before you buy it, make sure it runs/is not worn out, and get the serial number off it.
The 900 with a serial number #1000 to #4931 use a steel ball bearing indexer in the center rod shaft slot that would wear out the shaft. The 900's from #4932 onward used a bolt with a sleeve on the end of the indexing bolt, and instead of the center shaft wearing out via the ball bearing (expensive part), it wears out the index sleeve instead (few dollars to replace the sleeve).
Everything on the machine is replaceable, but just mention this due to the age of the machine, and what might be due for a replacement soon.http://www.reloaders.com/index.php?main_page=product_info&cPath=5&products_id=43
As for the early 900's, pretty sure that they came with the aluminum primer assembly like the later 900 model, and only the early 800CVT's had the plastic primer assembly (I'm tired and could be wrong through).
And FYI, the machine can be changed over to any gauge from 410 to 12 (including 16 gauge). A new tool set is around $200 to gauge change it (takes me about 5 mins), but used tool sets pop up in the EE from time to time used for about half that price.