A 4k display in "looks like 2560x1440" scaled mode gives you the same UI size as 5k in default resolution and is significantly better quality than a native 1440p display, because it isn't 1440p - it is 5k downsampled to 4k, which includes substantially more detail than 1440p would. Frankly, although it is slightly "softer" than a 5k display, it isn't night and day sharper. My 0.5c worth: if your workflow doesn't involve climbing up on the desk with a jeweller's loupe and doing A/B comparisons between the 4k scaled and 5k, you won't notice.
What you will notice is colour differences between the iMac's P3 display and a standard-gamut 4k. You can reduce that with calibration but, at the end of the day, a wide-gamut display can reproduce colours that a standard gamut can't.
The extra GPU load might cause lag on a Mac Mini or lower-end MacBook with with a relatively feeble iGPU, but The 5k iMacs all have half-decent GPUs and dedicated VRAM.
If you've seen the (awful) result of (say) scaling 1440p to 1080p on a 'non-retina' display, try and put that out of your head - MacOS scaling on "retina" class displays is in a different league.
Plus, it only takes a jiffy to switch it to either 1:1 4k resolution or pixel-doubled "looks like 1080p" mode to get unqualified 4k resolution when you need it. The main reason for using scaled mode is to get the size of system fonts, menus and dialogs "just right" - if you're displaying a full-screen image or even a window full of text that doesn't matter much - virtually every application lets you choose the magnification used for 'content'.
That said, going for 1440p secondary display(s) is a perfectly sensible way of saving a few hundred bucks c.f. buying (possibly two) 4k displays, given that you're mostly going to be focussing in the primary 5k, but I don't think you can justify it on "quality" grounds.
