Honestly, your weight should not be much of a factor in the coaming attachment. So long as the kayak is able to support your weight, the coaming should as well. You really don't put that much pressure on it. You might need to evaluate the aft deck as you will probably be sitting on it to get into the boat. Modern kayaks with a keyhole coaming make it easy to sit on the coaming to slide in. You can't really do that with a traditional boat. You have to sit down on aft deck to slide in. Take a look at the tutorial below. Notice where he puts his hands to slide into the boat.

http://www.capefalconkayak.com/usingyourgreenlandkayak.html

I paddle a Sea Ranger with a floating coaming ring and it is surprisingly solid, but I don't sit on it as I have found that those two inches of height difference really make things unstable.

Eric M.