"Mark is not suited" is not a noun phrase in the example sentence:
Mark is not suited to countries where cheese is available.
Indeed "is not suited" acts as the main verb of this sentence, so the subject is simply "Mark".
A good test for a noun phrase is "Can it be replaced by a single pronoun, and leave a grammatically valid sentence?" Replacing "Mark is not suited" in the example would leave:
He to countries where cheese is available.
That is not a valid sentence, for one thing it does not contain a verb. Replacing jsut "Mark" leaves:
He is not suited to countries where cheese is available.
That is a valid sentence.
Now at the other end "countries where cheese is available" is a noun phrase. "where cheese is available" is a restrictive relative clause indicating which countries are meant. such clauses can form part of a noun phrase. Again the replacement test leaves:
Mark is not suited to them.
which is a valid sentence.