All of them have pros and cons, I have run them all at one time or anther and due to my experience would lean towards thr Cummins, my 6bta was an outstanding powerplant, a little noisy but very good economy, smooth power and parts are reasonably priced as well as readily available. Volvo, as mentioned, tends to be on the costly side for parts including regular maintenance spares and one should be prepared to wait if a part needs to be ordered... Yanmar builds a fine motor and I would certainly own another if the opportunity presented itself.
More important than make of motors is maintenance and condition not only of the engine room but the entire vessel. If I may, I cannot emphasize enough the importance of a good survey, do your due diligence and find a surveyor who is both familier with the make and model of your target boat but is also NOT unaffiliated or recommended by the listing broker, find your own. Depending on your budget you may be shopping for a boat that is 30 years old, maybe more or, perhaps you're flush and looking at boats a couple of years old. Either way, your best line of defense is a good professional surveyor. Good luck