The complication that shows up most often here isn't the door - it's the opening around it. A house built in 1961 was framed for whatever door was standard that year, and sixty-plus years of settling, re-siding, and the occasional DIY patch job means the rough opening rarely measures square anymore. New doors are built to tighter tolerances than old ones, so a quarter-inch of racking that never bothered the original door will bind a new one.
This region sits in a cold-severe climate band, which raises a second issue specific to older homes: original frames here often don't have the depth for a modern insulated door's thicker panel without adding build-out strips first. Skipping that step is how you end up with a door that looks installed but doesn't seal, and then condensation forms on the inside face all winter.
The honest limitation: if the framing is bad enough - rot in the jamb, a header that's sagged - that's a carpentry job before it's a door job, and we'll say so rather than hang a new door on a frame that won't hold it square for long.