"Several thousand" is a bit of a subjective term, so to clarify, the upgrade of the infotainment system from MCU1 to MCU2 is currently $1500. You will lose the AM/FM radio, but you can get a new FM radio tuner for an added $500, so a total of $2,000 plus sales tax. This includes replacement of both screens. If you see bubbling around the edges of either screen, that's an indicated the glue seal has failed and the screen is leaking the gel between the two layers. This can make a mess, but my bigger worry is what's behind the screens in terms of electronics which the gel may be leaking onto. Again, this would be a good trigger to get the MCU1 to MCU2 upgrade as this includes both screens being replaced, so in my opinion, a good value versus doing a screen replacement.
If you have access to the car, you can easily confirm both the infotainment version and 3G vs. LTE modem by going to the software, then click on the Additional vehicle information link. If infotainment processor shows NVIDIA then it is still MCU1.. If it shows Intel, then it is MCU2. You'll also see directly an entry for the Modem capabilities which will directly state 3G or 4G.
Depending upon when the car was first sold, the 8 year unlimited mile battery and drive motor warranty could still be valid for the better part of two years (a 2015 produced car not put into service originally until 1Q 2016 would still have the 8 year warranty until early 2024). You would likely want to confirm details like if AP1 autopilot capability was purchased and enabled. It should have premium connectivity and likely may have free unlimited supercharging if the car has never been sold back through Tesla.
For the record, I do think there were some definite upgrades between 2015 and when the mid-2016 refresh models were introduced, but I know many owners that have 2015 vintage cars, continue to drive them, and had neither battery or drive motor replacement issues.
Of course all the other routine things you'd want to understand about any vehicle; does it have a clean title versus salvage, any history of major accidents/collision repairs, etc.
and FYI - my perspective is based from owning a June 2016 produced MS90D purchased new, that I've owned for going on 6 years. Since my 4 yr/50k warranty expired, I've paid just under $500 total for maintenance which was replacement/rebuild of two door handles. I did elect to do the MCU2 infotainment upgrade with radio for $2,000 in early December 2021. I do not consider that expense maintenance as it was driven by my choice to upgrade functionality, not initiated by a failure of any form. My wait to get it installed at my local service center was about 2 weeks, although I know some have had to wait much longer.
