Just generally speaking, a radial mount refers to a type of mount that bolts straight on to a flat vertical firewall.
Now, I'm going to try to convince you to skip the Astro and go straight to brushless. It's an older kit, meaning not 3D, and the price difference between a 150 Watt brushless and setting up that Astro 05 isn't going to be that much. The plane will also be lighter, and perform better. If they're expecting the plane to weigh 30oz with a heavy Astro 05 and a Sub C car pack, it'll probably come out closer to 24oz with a brushless motor and 3S LiPoly battery pack. If you don't want to go with LiPoly, you can go with a 10-cell pack of modern NiMH cells, and still come out way lighter. Plus, an outrunner brushless has its own radial mount.
150 Watts is going to make this plane utterly ballistic. I can feel it
An Eflite Park 450 outrunner is $65. An Eflite 20A brushless ESC is only $42. With a 3S LiPoly or a 10-cell NiMH pack, I'd recommend a 10x4.7 or 11x3.8 prop. For batteries, I'd recommend the 3S (11.1V) ThunderPower 1320mAh, the 3S (11.1V) Kokam 1250mAh, or a 10-cell CBP1100 NiMH pack.