$10.5k for a used engine? I remember seeing a 03/04 Cobra motor with tranny new in the crate in one of the Mustang parts catalogs for $6k a year or two ago. I would research more sources before jumping on this.
The V8 swap writeups will be able to explain the swap in greater detail, but at first glance you'd still be looking at cooling, mounts, and gauges on top of that setup.