Typical newbie. You embrace the illusion of transposing the sounds in your head to some idealized swiss army keyboard. That's why EVERY electronic keyboard made has transpose buttons. And the lowest octave and highest octave notes on an 88 board, depending on the voice, ranges from infrasonic to supersonic in frequency and that's only If the PCM samples were even created in the first place. If you play pop, or electropop, or industrial, or top 40 dance tunes those notes can't even be reproduced on most audio equipment and play distorted as hell.
Why don't you, tell us, what is your musical goal and genre and stop fantasizing about taking beginner fiddle lessons on a stradivarius?
It's more complicated than that. First, in regard to transposition, I don't know if there's some more automated method to that, but my thought process was trying to avoid having to stop playing, or having get distracted by the switching process, in order to to get that higher or lower octave, which is not ideal for this ADHD brain.
Sounds I would be using would be piano, strings, wind, a variety of synth like whatever I find/tweak on my sampling apps.
Genres would be instrumental EDM, pop, new age, folk, calming stuff for meditation tracks, background stuff for videos as I said. I've dabbled in this stuff before but never recorded and never had access to software or anything.
As I said in my original post, I did find a match for a "swiss army Stradivarius" as you would call it (an M-audio Prokeys Sono) for sale within my budget, so I was looking for advice on that and also if there are other possible options that may be better options to keep an eye out for instead.
Unfortunately I live pretty far away from any place that sells keyboards to try out and get re-familiarized with updates to keyboard technology that I've occurred within the past 20 years, and the only things I've ever played have been traditional pianos and my old 61 key Casio (toy) years ago. I realize I'm kind of flying blind here in many ways which is why I'm just trying to get as much technical information as I can that's applicable to me to make the best decision I can for either an online purchase on a used marketplace or to select a few local options I find in Craigslist and other local ads to spend gas money (on top of my $200 - 250 budget) to check out.