The answer depends on whether you can afford to eat the cost of a major repair. The i3 seems to be a pretty reliable car, at least compared to other BMW models, but every car will eventually break. BMWs, for whatever reason, cost a fortune to fix when they break, so this can be a really bad life event.
Let's take a concrete example. Let's say that, a year from now, the REx stops working (I'm NOT saying that this is likely. I have no idea how reliable the REx is, but it's possible). You will need to take it to a BMW shop to diagnose the problem. Let's say it's something major and the engine needs to be rebuilt. Hardly anyone is going to want to touch an oddball little derated scooter engine they've never seen before, so that means you'll be replacing the whole REx. I'm going to guess that the cost of doing that will be close to $10,000 at a BMW dealership, but someone please correct me if I'm wrong. Probably somewhat less at an independent BMW garage, but just the engine assembly parts will be five grand, brand new. That's a huge amount of money for a car that is worth less than $18k right now, and still depreciating.
Anyway, the point I'm making is that when you buy any used car in as-is condition (no warranty) you are taking a risk. Just be sure that you understand the potential risk and are prepared in case something goes wrong. Being "prepared" may mean having money in the bank to pay someone to fix the car, or having money in the bank to replace the car, or having the time and know-how to fix the car yourself. Just have some kind of Plan B.
My Plan B was having a CPO warranty and paying for a warranty extension out to 2023, but your Plan B might be something different.