Search for that word here and filter threads only, you'll find out quickly what's going on.
Initially these 75Ds would do about 5.1 seconds 0 - 60 but sometimes in 2017 Tesla started offering this "uncorking" upgrade and most (if not all) facelift 75Ds would qualify. I think it involved some hardware upgrade (contactors) and a new firmware configuration. Most of the used facelift 75Ds Tesla sells come already uncorked or it's solved with a quick service center visit. This is the ideal scenario. As you'll search, you'll run into threads describing a lot of pain with a car bough from Tesla, advertised with 4.2 seconds and then the buyer finds out that it's not possible to have it uncorked.
My 2016 75D almost had a similar story. Bought it from Tesla and sure enough it was the slow one (over 5 seconds 0-60). Two mobile tech visits and one service center appointment I was told it cannot be uncorked and basically directed to pound sand when I told them they advertised it as 4.2 seconds capable. On my next service center visit (different location) they happily uncorked it with a quick software update and it's all good now. Insanely fast. Neck hurting fast
So yeah, if you want as new as you can get, look for a facelift 75D, if you're ok with an older one, most likely a 2015 85D would be in the budget with some patience.
Since you mentioned you need free supercharging, that means private sale only (actually I think some 90Ds from Tesla came with free supercharging again for a while, not sure if this is the case anymore but anyway those may still be over budget) so then you'll be able to tell easily on a drive test if it's uncorked or not.
