From 2012 - 2020 I'm sure, they were Pop (basic), Easy (mid) and Lounge (top spec). In 2016 they changed the spec / gave the spec list a revision. No more red/blue/beige interiors, all black / grey with a bit of stitching details in grey or red. After this, you need to be careful as over the following years they started skimming down the kit. Speakers took a hit, 6 speakers becoming 4, 4 speakers becoming 2 in the basic models. The Lounge which was supposed to have leather steering wheel / gear knob also was being sold with just plastic parts but still charging Lounge pricing... The unique little 'space saver' handbrake also became replaced with the old standard level style. The skydome (glass roof), table folding passenger seat etc options that were all cool and unique were killed off too after 2016. The 4x4 was always based on the Lounge but obviously had the 4x4 hardware, I noticed the early ones also had fabric / material on the rear door cards which was a nice touch. Cross was even better with unique styling / lights and a 4x4 system with hill descent control / bit more capable.
4x4s I hear can be a bit problematic and very hard to find spares for the 4x4 bits but worth checking out the threads of owners on here for more on that
TwinAir engines... had the oil spec changed by Fiat a few times in the initial years. People feared they might not age well but they have proven to get to pretty decent mileage. The 1.2 FIRE is the one to go for imo, slower but legendary status / reliability / only issues are easy small things like rusty oil sumps, leaky rocker cover, etc but all cheap and easy to fix with auto factors having the parts same day usually. New Hybrid system, I don't hear are great to drive but nobody knows the long term reliability yet. But if you want a fresh / new example after 2021 it'll be your only choice.
With the Pop, just think hard if you can live without a button on the key. Sounds silly, but walking up to your car with shopping in hand, or friends already waiting by the car, having to wait for you to open it. Then take the key out of the car to open the boot (or to give to someone as they open the boot to get their bag). It might be something that you feel isn't worth the hassle, and I don't think it's easy to retrofit in any comparable way to the factory job.
Just look for a brochure and if you look at one used, check the brochure and make sure it has everything the brochure says. If not, don't worry, it's unlikely to be a botch job or had any parts stripped from any dodgy past, it's probably just how Fiat made it.
Pretty sure
@Wee Smurf ordered her yellow Panda back in the 2000s with a yellow interior but it arrived with a blue one!!! (or vice versa) - it's just Fiat up to its antics
I'm sure others can add to this, but above is what I've picked up over the years