Almost one year ago my wife and I purchased a 2014 Leaf SV with 61K miles and 11 SOH bars, so very similar to the car you're considering. When we picked it up from the private seller the battery charge was around 85% and I drove it >70 miles to get home. Note that over half of the drive was on surface roads, which in my area are 35-45 mph which will maximize range in a Leaf. I am pretty comfortable driving 35-40 miles from home (on 35-45 mph roads) without any concern that I will be able to get home.
My wife uses the car for her commute (about 32 miles roundtrip) and it just passed 66K miles and still has 11 SOH bars. She has free charging at work, so the Leaf is an ideal car for her. Even if the battery degrades considerably over the next few years she will have more than adequate range to do her roundtrip commute, even in the winter when the range drops a bit. Other than consumables (wipers, tires) our cost of ownership should be very low for however long we choose to keep the Leaf.
In my mind, this is the ideal situation for an older Leaf with <100 mile range - short commutes and running local errands. We have an ICE car (Honda Fit) that we use for longer trips.
So you first need to ask yourself if a Leaf is the right vehicle for you. Is most of your driving local or short commuting? What is your charging access? Do you have access to a longer range car if you need to drive hundreds or thousands of miles?
Regarding a replacement battery, let's suppose that you can get a replacement battery installed for $5000 (I have no idea if that is an accurate cost), and let's say that each SOH % equals 1 mile of driving range, so that going from 81% to 100% SOH would get you 19 miles of added range? Do you need that (hypothetical) 19 miles of range? Is it worth (hypothetically) $5000 to you? Remember that a new battery will also degrade over time, so your (hypothetical) 19 miles of added range will decrease over time.
In my opinion, if that added bit of range is critical, you would be better off searching for an EV (Leaf or other) or hybrid with a much higher starting range rather than making a large investment to replace the battery in an older Leaf.
