I am in the process of purchasing a 2015 model S and really don't know a thing about Telsa Model S what is it I need to know?
Generally speaking, buying a used Model S is a worse idea than buying a new Model 3. Again, generally speaking, Tesla seems to have succeeded in making people believe that the Model S is more premium and has more space while the Model 3 is the cheaper and smaller car. That's not the case at all.
When the Model 3 was released, the touchscreen, the touchscreen computer, the Autopilot computer were all more advanced than the S/X. Tesla upgraded the hardware in the S/X to match the 3. Similarly, the 2170 battery cells, the more powerful battery pack cooling, the digitally controlled air vent system, 180 kWh DC charging support, Track Mode, an audio system with much better sound quality are features that are only available in the 3. In the next few months, S/X might get some of these features.
If you compare the 3 to older Model S cars, then the advantages of Model 3 become more obvious. The touchscreen in S/X was upgraded to Model 3 quality on 14 March 2018. Autopilot 1.0 was introduced on 19 Sep 2014, AP2.0 on 16 Oct 2016 and AP2.5 on 28 Jul 2017. If you pick an old S, you will get old technology.
In terms of space, the Model 3 has more headroom than the S and the legroom is the same. The only advantage of the S is the additional cargo space. See the details
here.
In terms of range, the range numbers of Model S cars are over advertised. For example, Model S 85 has 265 miles advertised EPA range. However, the actual score is 244 miles, not 265. In addition, 244 mi is for a new S85. A used S85 will have less range because of degradation. See the chart
here. Therefore it would be much better to buy a new Model 3 MR instead of a used Model S 85.
However, if somebody already owns a Model S, I would suggest either waiting or switching to a Model 3 but I would not recommend buying a new Model S before 15 Jan 2019. I think Tesla will announce new Model S features between 5-15 Jan 2019. If that doesn't happen, then 5-15 Apr 2019 is likely. It is likely that Tesla will pick the first month of a quarter to release new features. AP1.0 was introduced in Oct 2016, AP 2.0 in Jul 2017 and the facelift version of the Model S in Apr 2016. All three happened in the first month of the quarter. This is not a coincidence. EU deliveries are finalized at the end of quarters. So they want to make sure they release new features after a ton of people have already taken delivery. Otherwise, people would refuse deliveries.