The Russian phrase translates closer to something like "By us, there is a motor" instead of "we have a motor". English doesn't really have a separate dative case to show possession, so there isn't a good direct translation. We need the dative pronoun (plural first person) though, instead of nominative, so "нас" instead "мы".
I am not sure what you mean by saying "to roll an r" but I try to help. If you are Brit you probably say R with your tongue located at the bottom of mouth. To say R like a Slav or Arab you need to keep it at the roof of mouth, and air which comes out from you need to make tongue vibrating. Tingue left and right side is sticked to roof of mouth, and top is vibrating like flag on the wind. Unfortunately is extremely hard to lern that if you are Brit. If you achieve that- you will be the man! ;-) if you want to practise I suggest trying to pretend a motor by trying to say very long rrrrrrrrrrrrrrrrrrrrrrrrr.
As explained in an earlier comment, the apostrophe is there to differentiate between ест (third person of the verb 'to eat') and есть (the infinitive of the verb 'to be'). The apostrophe represents the soft sign at the end of есть (ь).
This is a bit of a pain in the ass to have to remember as I normally try to avoid using any punctuation when using Duolingo; it would be good if Duo could drop this requirement and be a bit more flexible, but, as for now, that's how things are.