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I wouldn't worry too much if it's nearly new. All motors / batteries are warrantied for 120,000 miles / 8 years and the Model 3 reached the UK in late 2019 so 6 years left on that.
Best way to check health is to use teslafi but the previous owner has to have logged mileage with it already. You can also just charge to 100% and check estimated range on the screen but that's a less scientific way of looking at capacity.
Used Tesla... check / verify battery condition.
I've no idea how to do that, but have heard horror stories about the conditions of some and the costs to replace.
Nowhere near as bad the guy who blew his up because of the cost... but still, replacing batteries isn't yet a cheap thing.