There's quite extensive information on whats the difference between versions and when things came out on a web site I favour (so sorry for the number of links) as it seems to summerise the detail rather than wading through lots of posts on forums. From my reading:
- Free supercharging is only available oin new MS/MX and non Tesla sales of cars first registered before March 2017. Much more detail here:
Tesla Info: Do I have free supercharging?
- Autopilot hardware changed from the Mobileye system in late 2016 to HW2, and HW2.5 didn't arrive until 2018 way after transferable free supercharging went. The main changes over time are listed here:
Tesla model history and changes by year
- The difference between the different autopilot systems is often closer than you think. The early HW2 cars miss a lot of functionality thats available on the later cars, especially if you have HW2 and MCU1. As a result, unless the car can be upgraded, and for that you need to have FSD paid for, a HW2 car is not vastly different to the older AP1 system, and in some cases the AP1 is better (for instance it reads speed limits). On a long drive on roads like the French peage, both systems are more than adequate, I've driven to Italy and Spain quite happily in an AP1 car.. More details on the hardware differences:
Tesla AP, EAP, FSD, HW2, HW2.5 HW3, MCU1 and MCU2 feature differences
The differences between a MS and an M3 with respect to dimensions etc can be seen side by side here, although I think they only look at the current MS, the earlier cars had a 7 seat option if you class the 2 rear facing seats as seats, really they were just for kids and short journeys. The MS is noticeably larger and I think better looking but beauty is in the eye of the beholder. M3 wins on things like beling slightly faster to charge (although for the real speed improvement you need the new V3 superchargers and currently I think there is just one location with them in the Uk) and better efficiency (not that efficiency matters that much if you're charging for free).:
Tesla S3XY differences
The last could of points:
Depreciation - while Model 3s are still fairly new and not really suffered from depreciation yet, its an unknown element in the future. You would normally expect a used car to depreciate less than a new car as the depreciation is rarely a stratight line, a good £45k MS may therefore hold its value slightly better than a £45k M3 over say 2 years. But an MS with free supercharging is going to drop out of warranty soon except for the battery and motor so swings and roundabouts. There are more independant repair places setting up who can cost a fraction of Teslas prices (eMMC fix is £500 v £2500 at Tesla, door handles are a £80 fix inc labour v £350 at Tesla etc).
The MS would probably have free premium connectivity, the M3 might have a year free and then you pay £10 a month if bothered.
In your shoes I'd be looking at a facelift MS 90Ds are around 45k with what you want, or pay 5k and get a bonkers P90D (check it has ludicrous) - there are few 100Ds with free sueprcharging around and you seem to pay £10k more for them. Or I'd be after a M3 Long Range. I have a suspicion that the M3 Performance will be upgraded to a faster version at some point and take the edge of the current model, but thats speculation.