How potentially valuable is a domain that includes the exact keywords in the name?

Hi everyone!

I'm a complete noob at all this stuff and I was just wondering. Say I were selling a product, e.g. "electric toothbrushes".

1) If the domain "electrictoothbrushes.com" were available, would owning it give me a significant SEO boost?

2) If so, would this apply even if I don't live in an English speaking country and my domain ends in ".com" as opposed to ".com" + country ending? For example, I am from Brazil. "Electric toothbrushes" is "Escovas de dentes elétricas". Would the domain EscovasDeDentesEletricas.com be better/worse than EscovasDeDentesEletricas.com.br, which has the ".br" ending in Brazil?

3) What is the effect prepositions have on all this? "De" means "of" in Portuguese so, is EscovasDeDentesEletricas.com.br better or worse than EscovasDentesEletricas.com.br?

Thanks a lot!