Yes, it's great for you (and the environment) if you can save $3500 a year in petrol, but I would offset against this the cost of electricity in charging your EV to go an equivalent distance. What do you think the cost of this electricity would be for you?
I think the depreciation of an EV is a very large factor to consider because the price of a NZ-new EV starts at around $60,000, and you can pay $40,000 for a second-hand Nissan Leaf that's been imported from the UK. With no NZ Govt subsidies on the purchase of new vehicles, they are just too expensive when compared with equivalent ICE models, even considering the savings you mention.
As for replacing your battery, I would rather try to trade-in your existing EV for a higher range model if that is possible. I think you will experience less depreciation that way.
In theory it will cost a few hundred a year to charge an EV, but in practice, due to special EV rates for electricity in the Wellington Region, my entire power bill is decreasing. So I'll be saving 20-30$ a month in electricity and $3500 a year in petrol. Its strange but true. All electricity for me between 9pm and 7am is dirt cheap, washer, dryer, dishwasher, hot water heater etc.
Depreciation of vehicles is another issue, and worth considering. Operating an EV is cheaper than a petrol vehicle, even if you have terrible luck and replace the entire battery every five years. Is this offset by greater vehicle depreciation?
The answer to that is probably not, since a used EV with a new battery should be worth a fair bit of money (still more than a petrol car with same odometer, as it will be in better shape and much better value). In practice, who knows? We haven't got there yet. As a business you can write-off depreciation, but even as an individual it makes financial sense if you can afford the capital outlay because of the lower running costs.
I'm betting owning petrol cars is going to get more expensive in the future, and EV's are going to be cheaper. Its a guess, but in my vision of the future, there will be both carrots and sticks used by the government to increase EV ownership and I want to be on the smart side of that.
As for the prices of a new EV or a used LEAF, I just bought a ex-jap 2016 LEAF for 27k, not 40k. You would have to be hellbent on throwing money away to buy a 40k LEAF! New EV's are definitely expensive, but remember to add 15k to the price of the equivalent petrol car, thats at least what you'll spend in 4 years fuelling and servicing it.
Buying an EV is like buying a really nice petrol car and all your petrol for the next few years, all at once. Yes it costs more upfront, but you save a bundle in the long run. Because of cheap Japanese imports that have the Japanese government subsidies rolled into their price already, we get a fantastic deal here in NZ.
Note: You can now get breakdown insurance on EV's in NZ, cost about $350 per year and covers the battery. That should take some of the 'used car' anxiety out of it for some people.