Good Mileage on used truck

So I am new to this, but when I asked experienced people they told me either you buy truck with mileage significantly below 400K or above 600K. Don't buy truck when mileage is between 400K and 600K

The idea behind is that Truck will start having serious repare problems around 500K but after 600K if truck is running it was probably fixed. and under 400K you still have miles remaining to make money and then fix.


So what's your idea. I met two newbie owner operators who were happy that they purchased trucks at low price. Both trucks were close to 500K. In few days one truck already got check engine light and it cost owner 1500 to repair the other still runs without problems. So it might be somewhat true?

 

thanks