You do not want blisters on your feet at any stage of this trek to Everest.
Break in your Trekking boots. You do not want blisters on your feet at any stage of this trek to Everest.  Therefore, you want to be purchasing your trekking boots with plenty of time to get in a lot of miles in them.  You do not want to show up on a trip with a brand new pair of boots.  Nor do you want to have a pair of boots that are falling apart and well-worn.  Get your boots in advance and make sure they are comfortable, have good ankle support, are waterproof and have a more rigid sole for the trek.