...They use a Kubota block...
Just as a clarification, the Betas don't merely use the Kubota block, they use the entire engine. What Beta does is they "marinize" the Kubota engine with various components necessary for marine application (heat exchanger, raw water pump, etc).
We have some threads around here where we discuss the advantages of the smaller Betamarine engines as compared to Yanmar, etc. They both are good engines, but when I've compared the two in the smaller size range (2 and 3 cylinders), Beta has seemed to have an advantage in a few respects, including price.
On price, make sure you're comparing apples to apples. The Beta prices are usually for the package, i.e. including alternator, transmission, and motor mounts. Some of the other companies don't quote or list prices with the same bundle.
As for the Volvo quote, yes those are good engines but the universal complaint is that they are tremendously expensive to maintain due to the cost of spare and replacement parts.
I would think you could manage fine with one of the larger 3 cylinder engines.