Welcome and an interesting read. Looks like you scored a decent deal for that Mustang. Man it took a beating. But looks like you'll be able to get what you need from it. Lots of comments come to mind, but here are the bigger ones.
- I've done two Coyote builds, both with the Ford Performance Controls Pack. It would be plug and play with your engine. The one that would fit your engine (each generation is different) is M-6017-504V. There are aftermarket solutions available as well. I personally don't have any experience, but I've watched several builds where guys have tried to make the Mustang wiring harness and PCM work. Usually they give up. Apparently getting the PATS (security) function tuned out is pretty common. But you still need most of the components on-line, and even then can be challenging. Another big issue is the fuel system. The stock Mustang is a returnless system, meaning a special computer controlled pump, tank, etc. Ford converts this to a return style system for the crate motor which is simple and can be run with common in-tank or in-line fuel pumps. Good luck with this one. Not to be negative, but I'd predict that if you get it to work (that's if...) you'll easily spend the $1500 cost of the controls pack in time and effort.
- I assume you know for the IRS the only components you'll use from the donor are the center section (diff), and the knuckles/hubs. You mentioned CV axles in one of your posts, but the Mustang ones (stock or aftermarket) aren't the right length. The IRS option from Factory Five includes the proper length CV axles.
- The donor brakes
should work. I personally, haven't gone that direction, but many do.
- The Getrag MT-82 that's used in a Mustang is another area where you'll have some work to do. The shifter is way too far back, and requires some kind of mechanism to move it up to the proper location. I've seen some guys work this out with some fabrication. Just recently, there's a commercial option available, but it's not cheap.
https://www.formacars.com/store/mt82shifter.
- Related to this, the driveshaft on the Mustang is a 2-piece assembly that's of course way too long for the Roadster. But the bigger issue is the transmission end uses a different adapter than the slip yoke commonly used on the Tremec options. This has to be sorted out as well. Pretty sure Factory Five doesn't have any solutions or support for the MT-82 itself or the driveshaft.
- Going with a base kit will give you lots of opportunities for choices on things like gauges, wiring harness, pedal box, fuel tank, radiator, steering rack, etc. Factory Five of course has parts for all of these, but you could choose to go elsewhere. But one area I'd
highly recommend going only with the current Factory Five parts is the front suspension. Use their lower control arms (LCA's) and 2-piece spindles. Both are purpose built for these cars and are a big improvement over any of the Mustang donor options which used to be pretty common but aren't so much any more. The difference they make in how the car rides and drives is significant, including basically zero bump steer. Very much worth the cost IMO.
Good luck.