The argument about damaging a diesel
by running at low speed goes on forever. To paraphrase the Cummins/Yanmar guru on boatdiesel "I have never seen a recreational propulsion engine
damaged by running slow".
Having said that if you are looking at a new GB42 or whatever they are called today with twin 600 hp engines, the above statement may not hold at 8 kts. The hp required to push that boat to hull speed
is a tiny fraction of 1200 hp. If the engines don't warm up to operating temps, then you can damage them by running a long time.
The rudders on all fast trawlers are too small for slow speed use. At below 5 kts I used the bowthruster more than the rudder
on my Mainship 34T.
Don't know about stabilizers. Obviously they work better the faster you go. What I have read says they work ok at hull
speeds and cut roll in 1/3.
You are wise not to listen to boat salesmen.