FCPX, iMac 27" Retina 5K & eGPU ?

First, let me tell you: a 5K 27" iMac is a great choice.

Now, let me try to address your concerns:

eGPU: you can always buy it later, if and when you feel the need or use for it. At the outset, I dare say that, since you felt the need to ask, you don't need it.

Let's face it, people can work with FCP on any mac in the current product line - even a MacBook Air, or a Mini, and these don't even have a discrete GPU. The performance on any 27" iMac will be vastly better than any of those.

If you get the Radeon Pro 5500XT with 8GB of VRAM it will be mighty good.

SSD: it's fine if you get your mac with 2TB, but consider this: the difference between 1TB and 2TB is $400.

You could put that toward external storage - which you are going to need, anyway. For just over $100, you may get a fast 1TB external SSD. You also are going to need hard drives for backup.

RAM: 16GB is fine, but again: you could get more bang for your buck by buying with only 8GB ($200 less), and buying RAM from OWC or Crucial. You could get 32GB, for a total of 40GB, for a mere $135.

(Note to others: this is only possible in the 27" iMac; the 21.5" does not have RAM slots accessible to the user)

10Gb Ethernet: do you have any devices that would use it? If so, by all means, spend the extra $100. If not, no reason to splurge for it. Remember, your mac has plenty fast io in Thunderbolt 3.

CPU: you did not say, but it is worth mentioning: go with an i7 or i9 version, not an i5.

I specced an iMac on the Apple Store:

3.8 GHz 8-core i7 processor

16GB of RAM

Radeon Pro 5500XT with 8GB of VRAM

1TB storage

Gb Ethernet (not 10Gb)

goes for $2,699

(or $2,499 if you go with only 8GB)

As a final word on eGPU: they may be theoretically faster (depending a lot on which precise GPU you get), but they only produce benefits for some use cases, and if the software does effectively use them. I'd say definitely don't buy one now. Many users have complained that they don't get the benefits they were expecting. And, as I said, you may buy one later, which may be a faster, newer model with better software support.

EDIT: I forgot one thing! The one thing I would personally pay more for is the Magic Trackpad. You may be a mouse guy, and that's fine, but Apple trackpads are excellent, and in my opinion more effective and ergonomic than a mouse. Of course, that is a matter of personal preference, but one that I feel strongly about.

The 5K refers to the screen of the iMac. It has nothing to do with recording of editing ideo in 4K.

That advice you got only shows the ignorance of the adviser.

The gorgeous 5K screen on the 27” iMac will, among other things, allow you to view every single pixel of a 4K video (together with the rest of ghd FCP interface).

The 21.5” iMac has a 4K screen; the, obviously larger, 27” model has a 5K screen.

Both can edit 4K video (as can my 6 year old mac, for that matter; only more slowly)

You can save some money and go with a 21.5” but not for that silly reason. And remember, in the 21.5” you have to get the RAm you need, you can’t buy it separately and install it yourself. If in a year you decide you need more you have to take it in, and have an authorized shop nearly dismantle it to add RAM.