[NAME]  The Xe can support up to 8K displays on systems that have the necessary ports to run it, so yes QHD 75 Hz is fine. I didn’t realize that was possible to achieve over HDMI 1.4, but if it is, I’d go that route to keep your USB-C/TB4 ports open for other purposes if ever needed. If HDMI doesn’t work out, go with a USB-C to DisplayPort cable. Scaling issues would depend first on your scaling settings. If your 5520 has a 15” 1080p built-in display, then you may find that using that at 125% scaling is workable. And if so, that same scale factor might be usable for your 25” QHD display. In that case, any scaling issues would be limited to issues inherent to running any non-100% scale factor. If you want to use different scale factors on each display, then as long as they’re still fairly close, which they should be given your display setup, then issues or noticeable scaling issues should be kept to a minimum. If it bugs you though, you’d need a different setup. If you use your built-in display at 100% scaling, then a 27” QHD display might be a better match.
I plan to buy Latitude 5520 (i5-1145G7, 1x16 GB RAM, Intel Iris Xe Graphics). I intend to use it for office work, no heavy processing, no gaming. I would like to connect the laptop with an external monitor and use it simultaneously with a built-in monitor (1080p). The external monitor is 24'' QHD 2560 x 1440 @ 75 Hz and has HDMI1.4 and DisplayPort 1.2.  Will the laptop support this resolution and refresh rate of the external monitor (connected via thunderbolt4 or HDMI) and built-in monitor at the same time? Could there be any scaling problems on both monitors (using OS WIN10)?
QHD 75Hz external monitor on Latitude 5520