Leaving it plugged-in for long periods is what made the battery on my old MBP swell up, which meant half the machine needed replacing. I also see reports here of problems with overheating with MacBook Pros being used while connected to external displays. How do people mitigate these potential issues?
A lot of the stories you've read about MBP batteries swelling up / degrading health prematurely when plugged in constantly (especially when compared to much fewer battery cycles being used) was starting from the late-2016 MBP re-model (where they introduced the Touch Bar).
About a year ago (I think), Apple added Battery Management Software to macOS Catalina where it would try to learn from your charging routine and pause your battery charging at 80% and not finish charging until you fully needed it.
With macOS Catalina 10.15.5, Apple is bringing Battery Health Management features to the Mac for the first time, introducing the functionality on...
www.macrumors.com
There are also programs like Al Dente that are popular, where you can set a max % manually.
I've used both my previous 2016 13" MBP and current 2021 14" MBP for long stretches in closed clamshell mode into an external monitor (specially during the pandemic when I went full time work-from-home for ~1.5yrs). What I would do to try and be careful was at end of work day, put MBP to sleep, and then unplug. Next morning, I'd plug back in (so avoids being plugged in overnight). I'd also use it on battery "like a laptop" in bed or on couch during weekends (and evenings).
While it still remains to be seen how these new M1 Pro/Max batteries will be when plugged in (who knows, maybe they switched battery suppliers that are hardier when being plugged in for extended periods of time?), I think it's reasonable to assume that if you take some precautions (ie. don't leave plugged in 24/7, and use on battery "like a laptop" some of the time), you should be fine.
Also unsure about how to go about choosing a good match between the resolution of the new MacBook Pro and the resolution of the external display - rules of thumb, maybe?
The internal resolution of the MBP doesn't really matter when choosing an external display. Just get whatever external display size works for your desk setup and budget, and your MBP will be able to display the appropriate resolutions.
As
@pshufd mentioned, I think a great bang-for-buck is a 27" 4K monitor, which can be had pretty cheaply these days. At 27", running the monitor at "Looks like 2560 x 1440" will make for a pretty good size for text, icons, etc. And getting a 4K monitor will allow you to use macOS's "HiDPI" scaling feature. What it will do is pixel-double the "Looks like 2560 x 1440" to be like 5120 x 2880 (5K). It will then downscale that to the 3840 x 2160 4K resolution of your monitor. This looks better than if you just got a 27" 2560 x 1440 (QHD) monitor and outputted at the native 2560 x 1440. The only small downside is that your GPU has to work a bit harder because it's doing non-integer scaling, but unless you're doing something intensely graphical, you should notice zero issues (especially if you get one of the new 14/16" M1 Pro/Max MBPs).
I've used this exact setup for years (with a 5+ yr old 27" 4K LG UD-68 monitor), and love it.
