You can customize keyboard shortcuts with most linux ditros (and also Ubuntu that ship with Dell, or Pop!_OS which is a spin off of Ubuntu and ships with System76). I’d be more concerned about the applications that I may or may not be able to install.
So now coming to the hardware part. Battery performance is marginally better because the kernels are usually modified to suit the open hardware better. I’ve used the Dell XPS 13 for a few days and it works really well IMO. It even lasts as long as a MacBook Pro on a single battery charge with similar usage. I haven’t personally used System76 machines but I’ve heard some really good things about them, and are comparable to the Dell from what my colleagues told me.
I use only Linux distros on my HP laptop that came with Win10 and I’ve not had any major problems. The only issue i had was long back when the Wifi driver for the RTL8723 was misconfigured so I had to do it manually. Other than that even if you choose to install it on your older laptop, you won’t need to do any sort of hacks on the hardware or software.
My advice: get a laptop, try it out for yourself and see how you adjust before making the leap and spending nearly $1K for a well configured Dell or maybe more for a System76.