Questions on a 2001 Volvo VNL64T770

We are considering purchasing a 2001 Volvo with a 14.0L Cummins (VNL64T770) with 520,000 miles. The tractor has was used by a high end tradeshow logistics company and has been meticulously maintained and had a relatively easy life hauling lighter loads.

Being that this is our first tractor, I have a few questions...

1) What is the general feedback on this tractor and engine? Any well known issues?
2) What is the typical life span on this tractor before a complete overhaul is typically needed?
3) The dyno sheet came back with...

- 370hp at the wheels
- Oil pressure 40 psi HOT at full load
- Fuel pressure at 165pis at full load
- Blowby is at 8" at full load
- Ran for 30 minues at full load, let back down to idle and oil pressure was at 12 psi at idle

Is the idle oil pressure too low? What is the spec on that?

4) And finally a tractor in this condition with these many miles, what would it typically sell for?

Thanks again for the input!!