It's not majorly difficult, but it can be confusing, frustrating and time consuming as I've found out! I got a Pro's Pro Shuttle Express for Christmas and have done 14 rackets so far. My first one took at least 2 hours, now my fastest time has been just under an hour but I can feel that I'm learning more with each one I do.
The Shuttle Express was the cheapest and smallest machine which was important considering I don't intend on stringing very often, plus I can put it away easily when I'm not using it. For my requirements it does a decent job and the stringing jobs that I have done seem good enough to me! It's not the best finished machine in the world, and sometimes I think fixed clamps would be useful, but for an entry level machine I'm happy with it.
watching other people stringing is how i started. at that time, there were no youtube or anyone to teach, just hang out in badminton stores, observe and go home and experiment.
watch the longer video to get a feel on how the whole flow works. there are also some specific tutorials, some are more advanced, some are not.
it takes a while to really get a smooth flow. buy the thinnest and most fragile strings you can find (Ashaway Zymax 62, BG66, etc) and maybe you will get to string a new racket every 2 weeks if you are relying on your own playing.
I own a Pro's pro shuttle express and would recommend you spend a little more and get the Pro's Pro challenger 1.
The PPSU is a nice little machine, however it is not very newbie friendly (crank machines have the highest deviation in tension as it relies on very consistent pulls). Additionally, it requires frequent calibration in my experience. If you're experienced and string often, cranks can be very good and fast. However it requires lots of practice to make good stringjobs.
I purchased a Gamma Progression 602, 6 point ratchet dropweight machine and I find it much more user friendly. Very nice stringjobs and the ratchet system is SUPER easy to use. Of the ones you listed, I'd get the Pro's Pro challenger I. It's got a ratchet drop weight system like my Gamma and fixed clamps. You can't get much better for a beginner machine in that price range.
I learned to string from this forum and youtub, I've read every pages in this section, because I found my former stringers very inconsistent with their works, either the tensions vary from time to time, or they didn't straighten the crosses. it's a hobby or a second side job , who's know in the future? plenty of infos here for you, and the feeling of stringing a perfect racquet is so good. later on you can try other patterns , tying knots....experiment with new things. Finally good luck and welcome to the club.