I think 2013 onwards and with the best battery SoH you can find in your price range. Age and mileage really come second to SoH with the Leaf IMHO.
I got mine at 11 bars and around 80-81% SoH, had it for a year lost 1 bar and dropped to 77% but actually really didn't notice much difference in range or daily usage. Was still managing my fornitely 90km round trip about the same.
If you can afford it though a 2016 or newer with the 30kwh battery is a thought, with 77-80% SoH they are still as good as a 24kwh leaf with 100% health.
I enjoyed my 12 months with a leaf, but I've sold it and gone back to a hybrid. I'm waiting for the day when an electric car with at least 200-250 km range can be found second hand in the $15K-$20K range before I try again. 100km is perfectly fine around town for my daily use but it's not enough for other things I like to do on occasion. Meanwhile my toyota corolla axio hybrid is on track to do 800km on it's first tank of petrol since I bought it, really not such a bad compromise so far.