Learn Farsi: چه کوهستان سرسبز و پهناوری!

In Farsi exclamations starting with چه (what a), the indefinite -i suffix always attaches to the very end of the entire noun phrase. Since سرسبز و پهناور (lush and vast) acts as a single descriptive block connected by و, the -i drops onto the final word of that block.

Do I need to pronounce an ezāfe after کوهستان even though it is an exclamation?

Yes, you must say koohestaan-e sarsabz. Even though the phrase is wrapped in the exclamatory چه and -i structure, the noun and its adjectives inside that wrapper still follow the standard Farsi linking rules with the invisible ezāfe vowel.

I learned that کوه means mountain. Does the -ستان suffix in کوهستان make it plural?

No, the regular plural is کوه‌ها. The suffix -ستان (-estaan) means 'place of' or 'land of'. So کوهستان literally means 'a place of mountains', which we translate as a mountain range or a mountainous region.

How does the adjective سرسبز get the meaning of 'lush'? I know سبز means green.

سرسبز is a compound of سر (head or top) and سبز (green). When used for landscapes, it implies the area is green all the way to the top or completely covered in greenery, which is why it means 'lush' rather than just the basic color green.