In addition to the normal mechanical things one checks when buying a ten-year-old car, especially if exposed to salt in a winter, there are a couple of things I would do:
1. Get an OBDII adapter and one of the apps which tells you what the battery capacity is, as I believe this is the overriding concern at this point in history -
2. Find out if the Onboard Charger
(OBC) has been replaced. If it hasn't and you're in North America, find out when the i-MiEV was first sold (should be with the paperwork for the car, but you can also call a Mitsu dealer and give them the VIN and ask them) as the warranty for the OBC is 120 months from date of sale. The original OBC now has a high probability of dying and which will entail spending some time and $ to repair or replace (not by Misu). The OBC replacement cost is a prohibitive ~$5000.
Also, be prepared to replace the 12v battery if it has not been replaced recently.
You might note the difference between SE and ES
We also have a fairly old thread about buying a used i-MiEV
and there are also quite a few posts with queries about the same topic.