My Wireless is completely "greyed out" and Bluetooth is stuck with "spinning wheel". In Settings: General>About> Wi-Fi...

Overheating can be caused by several things. The first thing you need to do is figure out where the heat is coming from. No air flow means no heat transfer, so figure out where the air vents are. Are they blowing hot air, or is there barely a breeze, even when the fan is overtaxed? Most commonly, an accumulation of dust in the vents and fans through the cooling channels will be culprit in restricting air flow. Cleaning it out will work best. Turn you laptop upside-down and look at what you’ve got. Make sure your laptop battery is maintaining charge. Bad/Old Batteries generate a lot of heat. I would also recommend installing CCleaner on your laptop to ensure that software isn't the cause, forcing your computer to work much harder than necessary. Scan for junk/unecessary applications and files and get rid of them.

This model has a proprietary PCIe connector for an SSD. If the SSD is not installed at the time of purchase, the connector still is present and later professional or self-installation is possible. The problem may be in locating a "generic" version of this proprietary drive. OWC has been among the first to offer solutions for previous Apple Proprietary drives. It's not clear to me if they have done so as yet for the iMac. Please read: Can I add a SSD drive myself? with similar questions/ideas for you.

I'm having identical issues. In Settings: General>About> Shows Wi-Fi Address is N/A. Bluetooth however does show a MAC address but when I access Bluetooth from main Settings Menu, it keeps showing the spinning wheel of doom. I've seen numerous posts and videos stating that its the Wi-Fi module itself. Have seen some videos completely replacing the chip all-together, and others saying it can be from the soldering coming loose under the chip and to just heat the chip as @xterminal suggested. Can you remove and replace the entire module by re-applying RMA-223 paste? Or do you have to solder the chip back onto the board? Any suggestions, information, or advice would be greatly appreciated. Thanks.