Nice boats with limitations, like all boats. We have two 20s in the fleet, both with Yam 115s; as well as two 18s, and have owned other Alaskans and SSVs over the years, so we have a decent basis for comparison.
If you're committed to the 20, get the console. There's about an acre of open space in the boat; and the console will free up space to let you carry more weight aft, where it belongs. Also, the aft third of the boat is very wet in any kind of quartering sea.
I would consider either the 20 or the 18 underpowered with a 50 if I was going to do a lot of rough-water operation. On the other hand, the 20 in particular has so much lift built into the hull that you might be able to get away with it. I have had a 50 on an 18, and didn't like it for hauling heavy loads in the seas we encounter. The 115 on the 20 is overkill IMO -- just extra weight and speed that you never get to use, and it makes the ass end of the boat too heavy to power out of rocks without breaking something.
A two-stroke 90 or maybe even 75 would be just about right for the 20. I wish that was we had. If it were me, I would stay away from a tiller like the plague.
I have heard some reports of rivet failure/hull flex with the 20s (never with the 18s). It generally seems to occur when they carry lots of weight (i.e. 36V worth of trolling batteries and a MinnKota) forward. Go figure. They use wood in the transoms and floors, which will rot eventually. But, the project boat in the driveway right now is a 22-year-old SSV-16, floor's got soft spots but it doesn't affect the strength or utility of the boat any, boat's still as strong as the day it came out of the factory.
End of the day, the bare 20 is not significantly more seaworthy than the 18. We have come close to sinking a 20 as a matter of fact. On the other hand it is a lot more comfortable on a moderately to severely rough day; and can carry a lot more decoys, people and gear.
In that size/class of boat, I believe one would be better served with custom welded deep-V plate boats -- UNLESS you are trailering/launching at marginal sites, pulling the boat on and off a rocky beach, or other application where light weight and shallow draft are important (all are critical concerns for us). No other commercial V combines the light weight, strength, and capacity of the Alaskan (for example, the crestliners can't carry enough motor/the sea arks etc. don't have enough bottom/the starcrafts are too heavy)... and none of the custom builders seems to have a time-tested 20' hull that weighs less than 1,200 lbs., except the jon boat builders.