I did a bit of research before i bought mine, (a 12v Excursion with 46lb thrust - German Import from ebay or amazon, cant remember which) good enough for a large loch, and can still shift a 19ft Clinker against the prevailing winds and current, never let me down, but never really tested in anything stronger than gusty weather if you know what i mean.
Essentially you first need to consider the size of boat/people/weight you need to shift, but what stood out was that regardless of motor output, the battery is more critical, go for a good quality leisure battery with a high Ahr. otherwise, standard car batteries will give you about 30 mins of top end thrust before they start to die rapidly on you.
Also key is looking after the motor, they dont like weeds or stray leader, if it gets tangled in anything dont try and reverse thrust out of it, stop and remove as much of the offending material as you can, trying to motor out of it will only burn out the outboard.
happy shopping.