Swapping the door panels on a standard 1990s-opening is within reach for someone comfortable with basic carpentry and two people to lift. Setting the new torsion spring correctly is not. A torsion spring under tension is the single most dangerous part of this job - it's the thing that puts people in the ER with broken fingers or worse, and it's also the part that determines whether the door balances right for the next fifteen years.
Our honest limit: we don't sell you a door and walk away from the spring and cable work. If you want to handle the panel and track yourself and have us just do the spring, opener, and final balance, that's a conversation worth having before you buy anything - some doors come pre-hung in a way that makes a split job harder than it sounds.