After that I started mounting the motor for the Y-axis belt, and attached what I thought was the cog for driving that belt. While doing so I noticed that the cog (or pulley, as it's called I've since learned) was a bit smaller than the one in the drawings, and there was only one in the set of plastic parts I'd gotten. Since both the X and the Y axes are driven by belts and the extruder is also driven by a cog I'd expect three cog-wheel -thingies.
I contacted the guy who sold me the parts, and he elucidated: It turns out that driving the belts with printed pulleys is old-school and to error-prone and imprecise for good prints. Therefore the guys in the know have started using machined aluminium pulleys to operate the belts. He'd mentioned that I need to purchase belts and pulleys, but I didn't understand that at the time.
So, I've ordered some more parts from xyzprinters and will mount these as soon as they appear. I've been slightly disappointed with their delivery-times earlier, but now they've apparently switched to a new deliverer in Norway, I hope these ones are quick!
The guys in the know (I really must start hanging out on the irc-channels more) also told me that building the Prusa with a lower and an upper bed is no longer the standard way of doing things. With the printed parts I've got I can make do with just the upper bed, as there are some tall brackets that can hold the belt securely without the lower bed. The things one learns...
I've also finally gotten the hot-end delivered from hotends.com, that I ordered when I started this journey back in the beginning of May. It took 2 whole months to get here - but arrived in one piece and given that I've not yet needed it I don't much mind! Here's a picture of it on the packaging it arrived in:
