I'm surprised that they said the 1602 doesn't come with insulation. That is news to me. Maybe they changed the jacket design or eliminated it to keep the cost down.
To be honest with you, the Clayton kind of sucks for anthracite coal burning, unless you only want moderate heat and burn times.
Where the Clayton does shine and perform very well, is for burning hard wood and bituminous coal.
So, my recommendation is to get the draft kit and change your fuel to a mix of hard wood and bituminous coal. Then you'll get the temperatures and the burn times you desire.
I usually line the sides of the fire box with split hard wood, then load between the logs heaping with bituminous coal. I set set the ash door draft, turn up the wall thermostat, turn the combustion draft fan on, and enjoy the heat. This only works with bituminous coal. Anthracite just forges into one big clinker. The bituminous coal smokes some, turns into coke, and then into a lot of ash powder.
With as much success I've had burning anthracite coal in a Clayton, it is my opinion that it isn't well designed to burn anthracite coal. The Clayton does and will do well burning wood or bituminous coal, because it's design favors good burning of this fuel.
To answer your question on how much coal it will hold, I have found that the Clayton 1600 furnace will hold about 120 pounds.