Bob, mine came with the 15 horse, so I didn't think about motor size.
For trolling, or for lakes in Maine restricted to less than 10 hp, I can push it with the little 3.5 hp from my sculler. With that it probably tops out around 6 mph.
With two people and fishing gear, the 15 is just fine, and I could probably get by with a 9.9. With me, my wife and a day trip load of fishing gear, it tops out around 20 mph on a flat day. For three, or two men, a dog and a pile of decoys, a 20 might be a little better. It can be hard to get up on plane with the 15 and a full load, though it's doable with some weight shifting.
If I carried 3 very often, I'd be looking for a 16 with a 25.
The Lund Alaska's are a very common boat here, quite a bit more seaworthy than the Lowe. In 16 with a 20-25, they're a great lake boat; the 18's with a 30-40 are plenty of boat for anything but stormy days or offshore. Some sea-duck guides here use an 18, and many hunt out of the 20's.
Friends of mine co-own an 18 foot Lund with a 4 stroke 40. IMHO it's more motor than the boat needs. It flies with 3 men, a dog and decoys, and has been fine with 4. It handles whitecaps and waves to about 3 feet quite well, although it can be a wet ride for folks in the bow.