This is why I just don't care about my keyboard, mouse, monitor etc beyond a baseline of minimum comfort.
Typing at an extra 15 wpm won't make a lick of difference in how quickly I produce a product, nor will how often my fingers leave the keyboard or how often I look at the screen. Once I've ingested the problem space and parameters, it all happens in my head.
I often feel that having a "comfortable" keyboard/mouse/monitor is more important than a fast CPU or a fancy graphics card - just because of that slight extra feeling of pleasure/ease that lasts all day long :-).
The advantage of them is that my monitors and keyboards usually last a long time so putting money into them is not as wasteful as putting it into some other components.
One thing that surprised me though is that I recently bought a KVM to switch from desktop to laptop instead of a second monitor and this turned out to be both better and much cheaper. I gave away an older monitor to a relative and found that not having to turn to look at a 2nd monitor was actually nicer. Initially I really didn't want to do this and really wanted another screen but I had to admit afterwards that 1 screen + KVM was better for me.
RAM and disc space just matter up to the point of having enough so that I'm not wasting time trying to manage them to get work done.
It was a very cheap thing off Amazon. I'm in the UK so you might not have it - the brand name is "VPFET KVM Switch 2x1" and it has 4 usb, 1 HDMI outputs and 2x(1 HDMI,1 usb) inputs.
It's the cheapest in their range, I think (about £30) - they have better ones.
Not massively flexible. Has a clicker switch which you could put on the floor if you wanted to let you flip displays. Not super fast at switching.....but it does the job for me. YMMV!
I use a 32-inch Viewsonic monitor with this. It's the most expensive monitor I've ever bought but it's nothing special when you look at what's out there. I't just lovely to use. :-) I think a purist would complain bitterly about refresh rates or whatever but I just love it and I spend my time reading web pages or code or watching the odd video.
When I'm writing something from scratch in a few months I can bash it all out on a small laptop - it is (as you say) all in my head, I just need to turn it into working code.
If I'm faced with some complicated debugging of a big existing system, or I've inherited someone elses project, that gets much easier with a couple of giant monitors to look at numerous files side by side - plus a beefier machine to reduce compile/run times as I'll need to do that every few mins.
You may care more about picking a keyboard & mouse/trackpad/trackball/etc if/when you start to experience pain in your wrists/hands and realise the potential impact on your career if it worsens! Similar situation with seating and back pain.
Typing at an extra 15 wpm won't make a lick of difference in how quickly I produce a product, nor will how often my fingers leave the keyboard or how often I look at the screen. Once I've ingested the problem space and parameters, it all happens in my head.