Our December 2018 Performance 3 has an 81xxx VIN so the one you found could be a lot newer than you think. Hard to tell since they don't build them in VIN order. See if you can check the build date on the driver's door frame.
my November 15th 2018 manufactured Model 3 performance has a vin of 106XXX. Interesting to me that your december made vin is a lower number (picked up 12/04/18)
Also interesting is the fact that my current car is the second one. since I rejected the first one when I went to go pick it up. The delivery center had another build of exactly my car build (not hard with model 3 limited options, especially on the performance model) and they offered to swap me into that car, which was pretty much flawless (and the one I have now).
The first car had a vin of 144XXX and a born on date of 11/21/18. the second car had a vin of 106XXX and was born on 11/15/18 (I asked the delivery specialist for the specific born on date.
OP, in early december of 2018 my total bill for my model 3 performance was 78k (including EAP, destination, taxes). If I could get the same car with under 4k miles for 55k I would jump at it, provided it was in good shape visually. Thats enough of a discount for me to consider gently used. Only question I would have is, why is it there? Was it a return, demo car and if demo car why did it last this long? unless its fairly recent, then that changes things.