A forty will keep the bow up which is what matters in bad stuff.
Au contraire, especially if the boat's heavy and you're pushing the hull near the limit of what it can do. These hulls can hold a LOT of stuff. Takes a lot of go (and smart lading) to make it safe if you succumb to the temptation to fill it up with people and gear... Green water over the bow of an 18 Alaskan isn't as scary as it would be in a 14 jon but I still don't like it. Apply more power and that problem gen'lly goes away.
We have run SSV's and Alaskans in several configurations since the earlly 80s. Even the old SSV-18s were a little underpowered with the 40, IMO. A 50 represented a big improvement in pucker factor, a 70 2-stroke = even better, though even then I've tinkered a lot with props and such to get just the right combination, and I've still found myself in some situations where I felt underpowered. Disclaimer to above: I'm probably unnecessarily sensitive to power issues. I don't feel genuinely safe in a small boat unless it has enough motor to stand up on its hind legs regardless of how much weight I may be carrying.
Also: What year is the hull? They made a significant change in '98 or so and another one in the last couple years. The pre-98 Alaskans are skinnier/ all-around smaller boats than their later counterparts, much different in terms of ride and useful capacity, but still quite capable boats. And, you can more reasonably power one with a smaller motor.
Didn't see whether it was tiller or console. For my taste (and in my experience), the 18 Alaskan is awful damn wet to be running a tiller, unless you're talking the IPS hull.
Still: if it's a good deal, buy it. They're mahvelous boats. You may find the 40 gets it done for you... or you may wish to trade up down the road and do a repower, for which there are still several sensible options in this hp range. And at least around here, there appears to be a decent market for used 40s.