Used car buying fear

Hello

I have had it drilled into me from an early age, that cars above 40k miles, and older than 5 or so years are completely unusable, and should not be considered when buying a car.

I am going to be buying another car by august this year, and I am looking for something that “does the job”, but in a good manner.

The car I have taken a liking to, is the Mazda 3.

My parents are urging me to lease a new one for £300 with a £1k~ deposit, however I’m looking into used ones, as I see no need to spend that money, especially when I’ll never get a penny of it back.

Would a 2015-2017 50k miles Mazda 3 really be that unreliable? For reference, I drive like my Nan, and do less than 3000 miles a year..

Thanks for your input!