Trying to refinish that click-together decking sounds like opening a can of worms, tbh. I've installed some of it in the past, and the adjoining /adjacent pieces do not always sit flush to one another. Everything will need to be sanded down(with an orbital floor sander) to the height of the lowest slats, more or less. Those tiny slats of wood may just get pushed around by the power of an orbital sander - they are not fixed to the concrete underneath and they are attached to one another with a cheap spongy plastic. In other words, it may actually be easier to use a small handheld random orbital sander, on your knees, to quickly sand the surface of each slat (without having to sand down the adjacent slat to match). There's only a few thousand of them...
whatever finish you use will only treat/seal the surface. What about the back and sides? Before you start, be sure to flip a few tiles over and check for mold and rot.
You could maybe try power washing a small area, treating it with teak oil, and seeing if the results are good enough to make the whole sanding process unnecessary.
Try calling the bau und hobby again and asking them where the nearest place that rents floor sanders is.
Why do you want to sand it? As Pilatus notes, it's probably not going to work out well.
If they're just discoloured then a power wash, possibly with a scrub with oxalic acid (enka) will usually brighten the wood up. Then treat with oil, rather than varnish. BLO (Boiled Linseed Oil) is pretty good.
Coop rent them out, our local one in Glarus, we rented it a couple of years back, not exactly cheep though and you need to buy the sanding disk and foam holder thing on top.
Also, they are VERY powerful, it WILL throw you across the room the first time you turn it on!! make sure the room is totally clear of everything, also if you can seal the room off, the amount of dust these things kick up is unreal, so wear a mask !!
That's purchase. You could go cheaper, but the cheapo ones are usually less robust, so the wands etc wear out/break.
For what you're doing you don't want all that much power - better to use chemistry and multiple passes at a low setting than just blast the top 2 mm off.