If you prefer to buy, like me, buy one with REX at least, i have rarely used rex but with it I can drive any distance( should i switch job i dont have to change car!! ). The car will go at least 100 miles in before refueling or recharging in extreme hot/cold weather. I think that makes i3 Rex pretty future proof.
I bought a 2015 i3 Rex Giga+all options with 13K miles on it for little over $20K. I had dealer do the oil change etc. Tires looked good. Ones without Rex were about $2k less at least.
The next service is scheduled is March 2020, that i really like. So far only Oil sensor had been replaced under warranty(common problem in 2015 i3 rex) BMW sent a letter that they will cover that part for next 10 years/120K miles, which was nice surprise.
Pay attention to when your car was manufactured, after mid 2015 have less or no issues unlike 2014.
My i3 is model year 2015, its was first sold in dec 2015 and that is when the warranty started, so it will carry bumper to bumper warranty until 2020 dec.