Nice desktop – I have a couple of these and like 'em. Great with the i5-6500T CPU, which will handle pretty much whatever you throw at it. USB type C port on front helps with future proofing. Single DP video connector on back kinda sucks, though – this model has a COM port instead of the 2nd DP which to me adds little/no value and pretty much wastes space. Also 1-year HP warranty vs 3-year warranty for the new ones doesn’t give me any warm fuzzy feelings. If I got one, I’d immediately upgrade the hard drive to an SSD, bump up memory to 8GB, and replace the standard N-class WiFi card to the AC-class Intel card with Bluetooth (make sure to get the M.2/NGFF version).
The HP repair manual (http://h20565.www2.hp.com/hpsc/doc/public/display?sp4ts.oid=8376393&docLocale=en_US&docId=emr_na-c04828077) lists a DisplayPort adapter board on page 9. It appears that this can replace the serial port adapter board shown in the pictures here.
With a second DisplayPort, this becomes a more desirable system.
So far, I have been unable to locate a part number for the adapter board.
Have you tried contacting HP? They are the best ones to help you there.