Safety disclaimer first
I have a rope of 10 m which supports 3 kN (300 kg) strength. I will abseil with it as a single rope.
Edit addressing the new info about the rope:
Please don't use this rope for abseiling. It is not certified and it is marketed with a random selection of catch-words, no actual information about it properties except the diameter and supposed strength (which is useless in the absence of a specified procedure of how they arrived at that number).
Old disclaimer without specifics as the rope (still true, but more general):
I assume it is a static ("low-stretch") rope. With 3kN it is hardly a climbing rope (EN1891) - I am only aware of cords rated at such a low strength. If it is rated after intended load as often done in "industry" compared to peak/breaking load in climbing that aspect would be fine. If it is rated by max/peak/breaking load, then 300kN is not enough. The knot will further decrease this, meaning you won't have enough safety margin. In any case the question still remains: Is such a rope suited for the abseil you are setting up (rock contact)?
Actual answer to which knot to use
That being said the goto knot around here is a figure of eight. One reason is the same as for it's use to connect your harness to the rope: It is simple and easy to control. It also means, you don't need to introduce a new knot to a beginner. And it is reasonably simple to undo.
There's also the aspect of decreased strength of the rope when knotting it. That's not an issue with "normal" climbing ropes. Even the worst knot in that aspect won't bring the strength so far down, that it gets problematic. In general the smaller the radii in a knot, the more strength reduction (so bowline is better than eight better than overhand, all on bight). This is a gross over-simplification, knots are extremely complex and this won't hold true in all cases, it's just a rule of thumb. And again, simplicity to tie, inspect and untie are more important (that's why bowline isn't used for this around here - nothing wrong with using that though).