A few thoughts
I've been using Macs since the original 1984 128K Mac. I have *never* had a Mac desktop hardware failure. Not once. I've had lots of failures of external components: floppy drives, hard drives, mice, monitors, etc. I've had a few laptop failures. But I have never had any internal failure of any desktop Mac, and I've owned quite a few. I don't think you need Apple Care.
I tend to keep my Macs for at least four years before replacing them. I'm now using a 28" iMac with 8 GB RAM and a 3.2GHz processor. It is more than adequate for all my needs, and I use it for all the normal stuff plus programming and some simple graphics. My previous machine was purchased in 2008 and still works quite well; the only reason I replaced it was the need to move past OS 10.7.
You have a three-way choice between more RAM, a big Fusion drive, and a big monitor. First, always go for the biggest monitor you can get; you can never have enough screen space, and nobody ever died trekking from the upper right corner to the lower left corner of the screen.
You absolutely, positively want a Fusion drive; they vastly improve performance. However, you don't need to super-size it. My Fusion drive is 1 TB hard drive with 120GB of SSD, and it works perfectly. Remember, the only value of the SSD is for fast access to frequently-used stuff. You definitely don't want to be storing big files on it. It should hold the system files and the files used by your applications. The algorithms it uses for allocating SSD space are very smart.
The same thing goes with RAM. More is better, but there are definitely diminishing returns. The Mac OS is very efficient in its utilization of RAM; my 8GB are completely adequate to my needs. However, I'm pretty fastidious about closing apps that I'll not be using for a while.
The same thing goes with processor speed. Yes, faster is always better, but how often will you be tapping into all that speed? For example, suppose that you had a choice between two processors, with one being twice as fast as the other. How much benefit do you derive from the computer processing a keystroke in 200 nanoseconds instead of 400 nanoseconds? In many cases, a processor that is twice as fast simply spends twice as much time idling.
Of course, your needs are especially demanding: video editing is far and away the most resource-hungry application of any computer. This is especially tricky to judge because most video-editing applications are structured to deal with the limitations of storage and RAM. Without knowing the specifics, you're playing a guessing game. My advice is that you search out reviews of different combinations of hardware and software. Even that is problematic because most reviews are based on older hardware than you're considering. Your best option would be to try out a video editing benchmark test on the different machines at the Mac store.
Did you really expect to get a 3 terabyte SSD for the upgrade price of just $150.
