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I'm looking at a used hybrid Lincoln MKZ with 70k miles. The manufacturer provides an 8-yr/100k mile warranty on the hybrid system, so I believe I should still be covered until 2019.
I've read that replacing the battery pack in any hybrid can cost a minimum of $2500 + labor.
Is there some diagnostic that a mechanic can perform to tell how much longevity the battery pack still has in it, and if not, then to what degree do I need to plan the cost of replacement into my car budget?
At least Toyota has a special hybrid battery test that only the dealership can do. If you service your car regularly at the dealership, the test is performed annually and in Finland, this extends the warranty 1 year at a time to 10 years (with 350 000 km max). I suspect the actual test is performed continuously on the car computer and the dealership just reads the test results stored in the car computer's memory.
Unfortunately, I don't know if Lincoln has a similar design than Toyota.
However, typically traditional (non-plug-in) hybrid vehicle batteries are oversized for the job they do. All they need to do is to help accelerate the car to max speed. For example, my 2016 RAV4 hybrid has a 1.6kWh battery pack. This is 5.8 MJ. In comparison, accelerating the 1700kg car to 120 km/h (the max speed allowed in Finland) takes about 1 MJ, and about only half of the acceleration energy comes from the battery, so 0.5 MJ required. So, the battery is over 11x bigger than it needs to be! Of course, the battery is shallow-cycled with only 40% of its capacity being used, but even then it is oversized by a huge margin. Not only that, but also the NiMH batteries last surprisingly long amounts of time due to shallow-cycling.
Additionally, since the batteries typically last for the lifetime of the car, you can almost certainly find a cheap junkyard battery instead of having to purchase a new battery.
So, I wouldn't worry about the battery. I would worry about buying outdated technology when purchasing a used hybrid, as a newer car very well can have better fuel efficiency.
I spoke to two Ford dealerships and they both said the hybrid system itself performs diagnostics, and if there is no error lit up on the dash, the hybrid system is working fine. The two mechanics I spoke with at the two different dealerships said they each had only replaced one hybrid battery in the 10+ years they had been working there, and it was on the earliest hybrid models. One said the car had more than 300,000 miles on it.
As a side note, one of the mechanics told me cars with Nickel Metal Hydride (NiMH) batteries actually have a special battery reconditioning process once the batteries do wear out, but it apparently is a time-consuming process and requires special equipment.
