Fibre Store (fs.com) are a good supplier. A lot of their gear comes from their warehouse in Germany. They are a massive company. Well regarded.
If you are ducting the fibre you could look at something like this. This would be my economy choice:
Alternatively an armoured cable such as this provides more protection and could be buried directly, although personally I’d only bury SWA type cable. But this is a good universal choice:
I have also used Paragon Networks and their cabling is indeed excellent quality too, all British manufactured too. But dearer than fibre store. This is their pre-term steel wire armoured (SWA) suitable for direct burial. This would be my Rolls Royce choice:
Definitely only consider single mode, either OS2 or G657.A2 grade fibre. I would choose an absolute minimum of 2 cores, but for a little extra get 4 cores for flexibility. Allow some extra over on the lengths, it’s easy enough to coil up the excess. Consider what connectors. LC type is universal for SFP or SFP+ pluggables that go into switches etc.
I’ll expand a bit more on choice of network devices etc, later when I have a moment.