How are they for reliability
It's like asking how long is a piece of string. Any D8N up for sale at this moment will be at a very minimum 20 years old and much will depend on how it's been looked after. The one plus for you is that the machine has little or no electronics on it and the non-electronic 3406 engine is a gem, that is unless it's been re-powered with a C13.....
In 1992, I bought a used D8N for the company I worked for. It was several years old at the time, but had low hours. I have been told, that this machine is still in service, and other than routine, expected repairs, has been basically troublefree all these years. It is used a few times/week, to rip and push up shale at a clay mine in Arkansas. now that says something as to the reliability of these old D8N's. Of course, this machine has been well cared for, and has only had maybe 2 or 3 regular operators over all these years.
It's a 1989 and has a 3406 with diff steer and 4 barrel ripper 16500 hrs with a new undercarriage