ぜ is a sentence-final masculine assertive particle, more casual and rougher than よ. 侮り難い ('hard to underestimate') uses the classical -難い ('hard to X') suffix attached to the verb stem 侮り — a formal construction meaning 'difficult to do.' やら is a listing particle similar to とか but with a slightly older/literary flavor, often with a 'and what all' connotation. 見渡す限り ('as far as the eye can see') is a fixed expression — literally 'as far as one can look around.' The 人、人、人 repetition is a stylistic device for emphasizing quantity.