Had a quick look. Those engines don't match any of the other common marine engines that I know of. I suspect (but don't know) they are Hyundai's auto engines marinised. Both engines make quite a bit of HP for their displacement and I wouldn't expect either of them to be 'long lived lunkers' like you may be expecting from a diesel. Not a comment on Hyundai, just on lightweight marine diesels making plenty of hp from relatively small displacements. If you are a low hours user and maintain your gear well they may be great. If you do lot's of hours and expect your engine to run reliably for a long time, I would not put engines of this type in your boat unless they were seriously well proven. 

Regarding the various common options around the 250hp size there are the Volvo D4, but after owning 4 Volvos I will never own another one due to all sorts of quality, pricing, lack of support etc type issues. Then there are the CMD 4.2l range engine and they are IMO OK, without being really great unfortunately. Would rather have one of them than the Volvo though. Then there are the Yanmar BY series which are the marinised BMW engines. I believe that they had some early teething issues, but the 3.0l engine at the 220hp rating (not the 260hp rating) might be worth considering as I understand they are pretty good now. There is also the older 4.2l Yanmar based on the Toyota Landcruiser engine. I am not sure if they have a 'de-rating' down to ~250hp (315hp standard), but of they do I would consider one of these. They are a mechanical diesel, but apart from a period early on where they had valve issues (now long past), they are a good reliable engine. Another option would be the 4cyl 4LH Yanmar if you can get one (might be out of production). Mechanical injection etc, but well proven, reliable, and simple to maintain.  

After those you get into the Nanni's, Lombardini's, Yamaha diesel etc where the installed base in NZ is quite small. I think that some of the Nanni's and the Yamaha are also based on Toyota Prado and Landcruiser engines, and at least they are relatively proven bases.

 

Yanmar have had several crankshaft failures (I personally know of three one being the TECT Rescue), but Yanmar were quick to repair and sort the issues so +1 to them there.  Possibly could have been application related as well, can't comment further there.

 

Seasall are an unknown at the mo due to the fact that they are dropping some and going to the new R-series.  This is the new jobbie that has double-firing injector's which is how they get the stunning torque and power figures.  Unfortunately, they are definately totally common rail so 1ml of good old h20 will stop the engine and possibly produce a several thosand$ repair bill.  You pay's your money and takes your chances...

 

I think the Iveco's are of Cummins lineage, but further than that can't comment.  I do know that they don't do anything suitable for a trailer vessel under the 300Hp mark for the application you are considering, as a 7.5m boat will seriously struggle with weight at the arse.

 

You want the lightest diesel you can get, and include the weight of the engine mountings, stringers and ancillaries in the figures as well.  Be a big improvement if you can run a jackshaft and have the engine well fwd of the transom... 

My 5.9l Ivecos are from the joint development that Cummins and Iveco did to produce a new generation electronic engine 7 or 8 years ago. Iveco do a 3.9l 4cyl at 250hp which is a tidy displacement vs hp ratio if you want good durability. Trouble is you need a Konrad leg or similar due to the huge torque. Too many 'different' components to make servicing easy even though the engines themselves are pretty straight forward in my experience. The water comment vs common rail injectors is true, but I just run a set of racors in front of my Iveco water seperators, and water isn't an issue unless you get really lazy and ignore the warning alarms after forgetting your daily checks after getting bad fuel etc etc. The bigger concern that I have with electronic engines in a trailer boat is keeping the electronics dry and away from salt laden air.

Rob - you need to own a D3 (we have 2) to know how bad they really can be. I have talked to many other D3 owners and have yet to find one who would own them again. Just a few weeks back a person from this forum rang me for an opinion on some used (virtually brand new) D3's that he was considering buying to repower his boat. It just so happened that I also know the guy selling the engines and why he replaced them with Yanmars at considerable cost (basically it was cheaper than continuing to lose charters due to broken engines). The 'funny' part of this story was how I met the seller of those D3's. He rang me to promote his charter business to our company. When we first tried to subsequently book his boat he called back to explain that his boat was out of commission with engine problems. After he had spent a couple of minutes telling me the problems he was having I was able to say "you've got Volvo D3's in that boat haven't you". By the time we had finished talking he was much wiser about his engines than he was when he first called. The forum member potential D3 purchaser wisely went elsewhere.

As for buying a 2.4l marine engine producing 220hp as the latest D3 incarnation seems to be, that is similar to Yamaha retuning one of their 4.2l 250hp outboards to ~400hp, or my 5.9l 330hp engines being retuned to ~550hp etc, or in your case your D4 being retuned to ~340hp etc. My belief is that even at 300hp the D4 may not give the service life that some of the guys fitting them in heavy boats are probably expecting. Time will tell I guess. The history of marine engines is littered with good engines that got 'stretched' until they became grenades. Most manufacturers have done it at some stage, and some more than others. It probably even makes good business sense in spare parts and replacement engine sales if you can live with the angry clients. In the case of the D3 I believe that they didn't even start with a good engine for the purpose they are using it. I actually like the base specs of the D4 at it's 210hp (non-supercharged) rating, but after owning and getting to know 4 Volvo's much better than I should have to, I won't be buying any more green engines. 

When any engine gets past say ~3000hrs or 10 years of age I think that you can start judging it's reliability. After all, that is the life span that we ate least expect from a diesel engine isn't it? Once you have some age and hours on them the quality of the basic design and build becomes more obvious.

Most of these 'highly tuned' engines (maybe not the D3) run wonderfully when new and feel great. It is when they are 5+ years old and have maybe 1000+ hrs on them that you start finding out why stretching big hp out of a little engine (in some cases with dubious quality control) is not so smart.

Interesting to see Tagit's comment on the Iveco-Cummins development of the 4B motor, forgot about them.  Only thing I don't like about those new engines is what I alluded to with the comment that 1ml of h2o can stop them, it doesn't need to be in the engine either, you have an electronic issue or a sensor that gets damp and the computer will go into error mode and prevent you from operating the engine.

 

An 8.5m boat has much better load carrying ability, and space for internal components that are awkward shapes and high-volume than 7.5m which is where you will find issues with getting the boat to do what you want with diesel inboard power.  You need to look at a lot of similar inboard powered boats, sea trial them and then get your design ideas drawn up and see whether they can actually be built...