Ask them about their installs!
You get the most "bang for your buck" buying a pre-hung door. That comes with the door, frame, hinges, insulation & threshold. You will need the interior & exterior trim. You will also need shims, screws, expanding foam, caulk, paint and/or stain along with various sundries such as sandpaper, spackling etc. You may also elect to purchase a new handset.
Possibly hinges and hardware. They have an employee that works in the door dept that should be able to make sure you have everything you need.
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