Mook, welcome to the forum. Whereabouts are you located?
Great that you've researched the car and are comfortable with its original range and perhaps a 20% decrease simply due to aging. We have two i-MiEVs in the family and they admirably meet our daily driving needs which rarely exceed 50 miles in a day but occasionally go well over 100.
wmcbrine nicely answered your questions, but let me throw in a few comments as well, some of which duplicate his -
First of all, since you posted to the proper subforum I'm sure you've read some earlier posts on this subject. Here's another post that identifies the differences between the 2012 SE and ES:http://myimiev.com/forum/viewtopic.php?f=19&t=3228#p29071
Of significance for your consideration is whether the ES i-MiEV you are looking at has the CHAdeMO dcfc port - the only way you can tell is whether there is a lever on the left side under the driver's seat. You can also find out by entering the car's VIN into the Mitsubishi database (just replace the VIN in the URL with the car you're looking at to see how it was originally equipped:https://www.mitsubishicars.com/rs/file/monroney?vin=JA3215H16CU018997
CHAdeMO I found to be transformational for the i-MiEV, allowing much longer hassle-free daily trips than would otherwise be possible without this dcfc feature.
Regarding your particular car - low mileage is great as far as wear and tear on the drive components, but we don't know about the battery. For example, if it were kept fully charged and left out in the hot sun for months on end, it may well have degraded. One way to find out is to get an OBDLink LX or MX bluetooth adapter for the OBDII port and a free Android app called CaniOn and run your own test to see the Ampere-Hour reading of the battery. I would expect better than 35Ah, with 45Ah being the approximate max of a brand-new battery. Let us know whereabouts you are located as one of our members might be able to help you out.
Here's a rough check of battery capacity that is awfully dependent on how the car had been driven in its preceding 15 miles: the right gauge on the instrument panel has a number of different screens, selectable by the pushbutton under the display. Select the one with a left arrow at the bottom - that is Range Remaining (RR). Now, a rule of thumb that I use is that when the 'fuel gauge' (on the left side of the instrument panel) is at the halfway point (i.e., eight bars showing) then I would expect the RR reading to be somewhere around 32 miles. This presumes the car hasn't been leadfooted by the preceding driver(s).
To put my two cents' worth in answering your questions -
1. Mitsubishi dealer - what's that? The only time I went to a dealer was for either a recall or warranty repair way back when. Haven't been to one in years. Don't worry about it for now.
2. $6K would be an ok price IF the car has CHAdeMO. I'd still be leery until I found out what the real battery capacity is for a car this age. Price is also a function of location. Whereabouts are you?
3. Forget about the battery replacement due to degradation as Mitsu won't cover it and it will cost you much more than the car itself; however, if there is a serious battery defect such as a bad cell then Mitsubishi will still replace it for free under the 10-year warranty. As wmcbrine mentioned, do expect to replace the 12v battery if it's old as a weak 12v battery results in many mysterious issues with the car.
4. Forget about public charging stations as you may never use them unless you live in a multi-unit dwelling and don't have a cooperative landlord or HOA. Simply plan on overnight charging at home using a plain old 120vac outlet. Having a 'full tank' every morning and never going to a gas station is priceless! That said, the i-MiEV is equipped with the national-standard J1772
connector which means it can be charged at most all local charging stations.
Good luck and let us know how thing go for you.