Your question, "Is the cummins 150 hp reliable?" I do not want to lead anyone down a particular path with one engine over another as we all have our opinions. I can say I have been running the Cummins-Mercruiser 150 since 2010 and have only had to replace one part - a IAT sensor this year. Starts first time within about 4-5 seconds and runs quite nicely. I had to inject the Cummins-Mercruiser comment as rightly stated in a previous post Cummins and Mercruiser dissolved the joint venture several years ago with Mercruiser taking over service and support for the QSD series. I for one have no issue finding a part, if needed, as my marina is a certified Mercruiser repair facility with two crackerjack young certified mechanics. I have found the Mercruiser on-line technical support for engine questions to be very good. As any engine out of warranty, which I suspect the ones you are looking at are, service become a matter of dollars. I find what I refer to as common maintenance easy, impeller change about 45 minutes, oil and filter change about 45-60 mins, secondary engine fuel filter about the same. As with any engine (CMD or Yanmar) mounted in an R25 classic one must get into some funny positions to accomplish standard maintenance. The only special tool I have purchased for the engine was the ECM diagnostics cable as early on in the QSD life finding either a Cummins or Mercruiser service facility with appropriate cable to interface with their laptops (for diagnostics) could be a challenge. I found the part number and purchased one off of Ebay. Now it stays on the tug and always available no matter where we venture off to. Sorry for the long post but the question is somewhat an opened ended question as no matter what engine, Cummins, Mercuiser, Yanmar, Volvo, Cat, etc. they all can have an issue. However, typically with a diesel give it clean air, clean fuel, change oil, and use it (diesels do not like to sit) they will last a very long time. With all of the above I could arrive on the tug tonight or take a cruise tomorrow and have it not start or stop running as could happen with anything mechanical. I would say after a good marine survey go where you are comfortable.