Two questions -
1. The apron that covers the gap around the stove and fireplace opening - are these sold separately by Hitzer or they are generic from any hearth dealer ?
It comes with the standard 6" surround (two side pieces and a top piece). You will need at least 2" of coverage all the way around, meaning the surround will need to overlap the brick at least 2" on all sides. This is so you can have an adequate layer of insulation around the insert to keep air from entering the firebox from the living space. The insulation goes between the surround and the brick to form the seal. I've read that some 503 owners have even caulked the bottom front of the insert (where there is no insulated surround) to keep air from entering there. I think they used high temp silicone caulk or some other kind of heat resistant material.
2. I got a Gold door - would prefer black - does anyone know if it is possible to change that
When I ordered mine, the gold door was a $172 option, and the brass trim for the surround was a $53 option. I thought the brass trim for the surround was too over-the-top visually, but I actually like the look of the gold door better (matches the brass around the blower exhaust vents), but I think my wife either didn't like it or thought it was too much $. I'll ask her again. If she likes it, maybe we can trade doors!
Keep in mind that the gold on the door is REAL gold. It will scratch VERY easily!