1) What is the difference between the Andexinger versions with S at the end of the model number and the ones without the S? The Andexinger web site is not very useful.
I don't think that there actually is
an Andexinger 486S, at least not according to the Andexinger website. I think that's a mistake by Thomann, or maybe Andexinger changed the name recently. According to the Andexinger site, there is only an Andexinger 486, not a 486S (but there is a 486A, which is the same as the 486, but with hidden height adjustment knob). According to the Andexinger website, the 486 has a seat size of 55x33cm, which conforms to what Thomann says about the 486S (with a slight difference: 55x32cm). Actually, other German online shops have the 486S too, and they
quote the size as 55x33cm, just like Andexinger does for their 486 (sans-s).
Now why am I fixated on the size? Because according to the Andexinger website, for the 484 there actually are
two versions: The 484 and the 484S, and those two do
differ in size (75x33 and 65x33 respectively), so it seems that the "S" suffix does indeed mean "small", at least for the 484 model. But since the 486 model only comes in one size, Andexinger lists it only as 486, but many retailers list the same size as 486S, which makes me assume that it's just a name mixup and they are otherwise identical.
2) I will not be adjusting the height very often, the bench is primarily for me. I don't need the convenience of automatic hydraulic/gas lifting. But if there is a difference in the risk of creaking, the stability, the durability that would be important. So will a classic bench with the two manual hand "screws" on either side be fine for me or are there advantages to choosing the lift-o-matic ones?
I have never tried the 486, but seeing the very good quality that Andexinger has, I would assume they they don't creak and are very stable.
I myself have the 484, (the lift-o-matic) and I am very
happy with it, even though I don't adjust the height very often myself (actually, once I brought it home I adjusted the size once and that was it). It is very stable (feels quite massive) and doesn't creak at all. What I like so much about the 484 is the size. At 75cm it is a full 20cm wider than the 486 (even the "small" 484S is still 10cm wider) - it really gives the impression of a real bench (not a stool) and, in a pinch, can even be used for 4-hand play (although not comfortably over longer times, but certainly for a bit of fooling around on the keys together with a child...). Note that the wider 484 cost the same as the smaller 484S.
3) Appearance is not very important to me, so I don't care whether the cover is leather or something artificial. But I do mind about the usability and the feel. I don't want to buy a bench with an artificial cover that is horrible to sit on, that makes me sweat and so on. Do I need leather or will the cheaper covers do fine?
I didn't want leather (or artificial leather) either, because I don't like sweating on leather in the summer (I used to have a car with leather seats and a leather couch, but no longer - for those I prefer a nice soft fabric too).
Fortunately, I was able to actually visit Thomann's brick-and-mortar shop and check out the benches in real life, to get a feel for the fabric (I already knew that I wanted the Andexinger 484, but wasn't sure about the fabric).
I can tell you that I didn't like the feel of the "standard" fabrics, the ones that Andexinger calls "Stoffe" on this page: http://andexinger.de/html/stoffe.htm.
But the ones that Andexinger calls "Plüsch" on this page: http://andexinger.de/html/velours.htm
are very nice.
Confusingly, while the page itself is called "velours.htm", the text on that page calls the fabrics "Plüsch". Well "Velours" is the french word for "Velvet" ("Samt" in German and Google tells me "fløjl" in Danish), while "Plüsch" would be very similar but maybe a bit less "soft" (Google tells me it's "plys" in Danish). Whatever the actual difference between "Samt/Velvet" and "Plüsch", the seat fabric feels very good, soft and, indeed, like velvet.
So I ordered my bench with the 25A Plüsch (which is black, not dark grey as the image on the Andexinger website might suggest) and I'm very
happy with it.
But for the 484, I actually had to call Thomann to order it with the 25A fabric, because there was no way to order it online. For the 486, I see that there is this variant of the bench on the Thomann site:https://www.thomann.de/dk/andexinger_486_s_verschiedene_farben.htm
where you are asked to specify the wood color and seat covering upon ordering (I don't know how that works though).
For the wood you could refer to here: http://andexinger.de/html/holzfarbtoene.html
and for the seat covers to here: http://andexinger.de/html/sitzbezuege.html.
Or you just given them a call (if you want to order from them).