I can tell you it’s late 90s model D5C Series 3 Hystat drive. I got the serial number today. It’s 6CS00829. I think the machine may be older because I am unable to find any diagnostics like you suggest.
It actually dates from June 2000 but it doesn't have the all-singing all-dancing electronic system on it that I originally thought and posted above.
You're going to need a fair bit in the way of test equipment unfortunately. See the attachment.
FYI the pressure gauges you need are 400kPa - 60psi, 1000kPa - 150psi, 4000kPa - 600psi.
You will be able to get by with one of each, just that the testing will take longer because you will have to stop more frequently and keep swapping gauges between test points.
Before you go further, when was the last time the filters were changed in the hystat.? it may pay you to pull the filter and cut it open to look for sparklies and/or shrapnel.