I have a July 2014 Model S. I've been pretty lucky with reliability but I have had a few relatively hefty bills over the 3 years I have owned it:
Original TPMS system died - it happened just before the annual MOT test (I don't think you have this in the US, it's a test you have to get done every year to check roadworthiness and TPMS failure is a fail) so I had to get Tesla to fix it on the quick, which was an upgrade to the newer Continental system. £1000 ($1400) - given a bit more time I could have swapped out the dead antenna for a salvage one for £70 ($100) probably but I just didn't have time to sort it.
Right hand headlight got full of water and failed £1000 ($1400) - in retrospect I would have done it myself, but the part is still £750 ($1000);
And a couple of smaller things:
Front left upper control arm bearing started knocking. Tesla said it was in tolerance still but it sounded awful going over bumps so I just bought the part and swapped it myself £250 for the part, probably would have been £500 with labour ($700 ish)
front left anti-roll bar drop link boot failed - again I got the part and swapped it out (just a case of taking the wheel off and undoing 2 nuts) - not expensive but shouldn't really happen on a car of this age, but given the same side as the control arm I suspect it may have taken a hit perhaps.
Wiper mechanism bushing wore to the point the driver side arm came up and was touching/wearing away the bonnet (hood). - a couple of hundred £ for the mechanism and a new arm and I swapped them myself.
Also my MCU screen has bubbles (common issue on the oldest ones - newer ones have a problem where a yellow band comes around them, which can be fixed with a UV treatment). Bubbles are caused by the adhsive holding the screen on melting in very high temperatures (which cars get on hot days in the sun) and there is a layer of liquid which is part of the touchscreen leaking out. when it gets bad the touch function doesn't work where the liquid is lost, plus the liquid which has dripped out is super sticky and makes a right mess I have heard. luckily mine is very limited but we don't have that many hot days here). the new screen/MCU is £1000 ($1400) and then there is the EMMC issue, which is a free fix if not already done. Older cars including mine probably only have 3G rather than LTE, which is about £500 to upgrade. If like me you are looking at screen bubbles and a LTE upgrade then the better bet is to upgrade to MCU2 which should fix both for £1500 ($1500 wierdly) and improve everything else.
If you have time, facilities and the ability to do work yourself you can save a lot of money, but parts can be pretty expensive too. The service manual can be found online, and the electronic parts catalogue as well, these are your friend if you have any problems.
my car is pre-AutoPilot, which some people wouldn't go for, but personally, having driven AP1, AP2 and AP3 loaners and testdrives I'm not that bothered, it's useful at times (on the highway) but super annoying at other times (I use cruise control for all speed limits and AP goes nuts and phantom brakes all the time on busy streets with parked cars to the point where it is unusable. The AP cars don't have a "speed limit" mode where you can set e.g. 20 and it not let you exceed that, or an old fashioned cruise control to do the same, without using AP which does all the mad braking).
I think back in 2014 you could have "tech package" which gave you puddle lights, a powered liftgate and a few others, but not really sure. in my experience everyone got this as you wouldn't really consider spending £70k+ on a car without these things.
I was careful when I bought mine (by private purchase) to get it with 6 months or so of full warranty remaining and I took it in before the warranty was up with a huge list of everything possibly wrong so I could get the whole car up to scratch before it expired. You may not have that luxury but it is worth considering, or getting one from Tesla with at least some warranty (if you do that though you'll lose free premium connectivity and free supercharging, but those things are peanuts in comparison to a significant non-battery or drive related failure in your first few months of ownership. Also to note is that back then the service experience was really good and they fixed everything straight up with no queries, I suspect nowadays its a different story but being out of warranty now (excl battery/drive which I have had no problems with) I can't confirm.
Note that the older batteries have been significantly throttled in terms of supercharger speed. it depends what sort of driving you do, but if you can't charge at home or you do lots of >200mile journeys then I would say it's a no for one of these (60, 70 or 85 batteries).