What would to bring and what to leave behind?
clothes (everything light is good because the weather is so hot)
Good to bring furniture or domestic appliances or not?
Domestic appliances could be brought but the cost of energy is quiet high. A cooking gas of 12.5l cost more than $30, and electricity is quiet expensive. Most, if not all appartments are furnished, no need to bring your furnitures. I case you need any, you can buy them locally.
Any foodstuffs that are banned in Rwanda? No
Better to bring a car or buy one in Rwanda?
Better buy one locally. You can have a good running second hand Toyota 4*4 for around 10k. Do not think other brands if you are still willing to bring your own car. Toyota is the most popular brand here.
Cheers,
David