I don't know what you mean by a Bentz engine. Having a Volvo D-12 in our truck, I can tell you that it has EGR issues. I'm told that Cummins engines have similar issues. Still others tell me that that, industry-wide, all engines of this vintage have EGR issues. I don't now how true that is. I do know that the Volvo D-12 has EGR issues.
Our D-12 has plenty of power and I am generally satisfied with it. The EGR thing was a disappointment that will add a penny per mile in additional operating costs going forward because the EGR valves will have to be replaced far more often than expected when the engine was first built.
The D-12 is now a discontinued engine, replaced by the D-13. Mechanics tell me the D-13 does not have the EGR issues the D-12 has. They would know by now since the D-13 has been on the road long enough.
Regarding the price of used Volvo engines, it matters which engine you are talking about. The D-16, now discontinued, was a disaster release for Volvo and major mistake. If you see one of those for sale, you would be money ahead to pay the sales person $1,000 to keep it. That engine has many people inside and outside the company shaking their heads and wondering how Volvo went so far off the reservation by putting a piece of junk like that on the market. They sell cheap in the used truck market for good reason, and that is a good reason for you to avoid it.