Of all the Lagoons in the 38 to 42 foot range, the 410 is probably my favorite as a nice blend of reasonable performance and accommodation. I've never owned one, but sailed several including one that a friend owned.
The engines under the aft bunks was never a big problem in terms of heat and sound and there are advantages to doing work
on the inside and out of the weather
. By and large the access for working on them is good to very good, with one big exception on the 40hp Yanmars, anyway. I learned this when my friend needed to change his water pump
impellers. At least on his (a 1999, I believe), the water pump
was located in such a way that you could not access the screws on the front plate to get to the impellers! We tried all sorts of ways including with an angled ratchet wrench. We could not get to them. We ended up having to jack up the engine
(taking it off the front motor
mounts) with a hydraulic jack to change them out. How frustrating! I don't know if they changed this in later years, hopefully so, but what a PITA for a regular maintenance
The second thing I don't like about them is the traveler. The endless loop design with undersized locking cleats
chafes the line excessively, then the cleats
won't lock, and the darn thing becomes pretty useless. If I were to own a 410, I'd definitely change the traveler to something more useful and secure. Fortunately, that's a fairly cheap
Regarding the lifespan of the engines, I doubt that having them under the bunk significantly increases or decreases their lifespan. I think there are many other factors that weigh more heavily on that (e.g., routine maintenance).