Kent, your plan appears sound but I have some cautions.
About Yanmar ... my '84 IB30 came with a Yanmar 2GMF - 13 HP continuous output. I think it was the standard engine for that year. It's a great engine -- it runs fabulously, does not burn oil, and its compression is still good a quarter century later. At my survey it started with 2 seconds of cranking after 2 years on the hard. In cold weather it starts after only a few seconds of cranking if I set it to 1/2 throttle.
However, there is a drawback. The 2GMF is a very tight fit! After I change the big Racor filter, I can't bleed the fuel line using the fuel lift pump w/o burning my arm when the engine is hot. I hope the newer Yanmar models you will be considering are smaller and better arranged. Be careful and thorough doing the research to ensure the new engine will fit.
I have also wished I had a little more horsepower when pushing upwind in some chop. If the rule of thumb is 2 HP per 1000 lb, my boat should have a 17 HP continuous output engine. Even now Yanmar offers models around 14 and 21 HP but not one in between. Finding something with that rating is possible. I see that Westerbeke offers the 18 HP model 20B2 (see http://www.tadiesels.com/west-20btwo.html)
. I've no idea how good the engine is or how easy it is to service, so don't take my mention as an endorsement. But if I were doing the research the 20B2 would be on my short list of engines to look at closely.