1.5" Super Isol board
thin layer of fireclay for perfect level?
I would use nothing. The Isol Board should be pretty flat already. So just lay the firebrick right over it. If you have a low point some sand, as mentioned later, can be sued.
For the number of brick if your oven is 48", then:
(4x3.14x(24x24))/2=3617" square is your oven dome surface.
Each brick on the small side is 4.37*2.68=11.7" square
3617/11.7=309 bricks required for the dome or 155 bricks cut in half
Floor radius is 24 (48"/2) + 4.5 (9"brick cut in 1/2) =28.5
Floor surface is 3.14*(28.5*28.5)=2550" square
Each brick on the mid side 9x4.37=39.3
155 for the dome plus 65 for the floor = 220
Please someone double check my math but I think you should have enough brick to do your oven and a landing area in front of your oven.