Remove 'it' word ...
My car is beautiful
The way you wrote the sentence is correct: Brazil is a beautiful country.
"My beautiful cousins" is indeed a perfectly grammatical phrase. It is, of course, not a complete sentence.
Yes, the sentence "Yesterday Tom washed the car" is grammatically correct.
Yes, perfectly correct.
Yes!
To correct the unclear reference, you must reword the sentence. The sentence isn't clear that the pronoun 'it' refers to the suit or the car. Reworded: Take the suit to be cleaned when you take out the car. Put the suit in the car and take the car to be cleaned.
yes, correct. Great job!
customers are going to purchase a car
Yes "She had a car" is correct in the sense that She used to have a car before but it is not there right now. It would be incorrect to the sense that She had a meal which means she ate the meal..
Past-tense. Solid wording.
Yes, very correct.
they're both correct depending on if your talking in past tense or present