Carhartt Duck Double-Front Bib Overall

Are these overalls lined, or are they unlined for year-round wear?

These are unlined overalls. They are ideal for year-round use because they allow you to easily layer underneath with thermals or hoodies during freezing winter months, while remaining breathable enough to wear over just a t-shirt in the summer

Do I need to buy a different inseam length than my normal pants?

Yes, many workers prefer to size down 2 inches on the inseam (length). Because overalls hang from your shoulders rather than resting on your hips, the crotch sits lower, which naturally pushes the cuffs further down. Buying your normal pant length often results in the cuffs dragging or bunching heavily on top of your work boots.