You’ve got excellent answers.
No need to worry a lot about malaria. If you are not used to it, high altitud might be your worse problem, but it is easy to deal with. If you go straight to Cusco, take it easy, very easy for a couple of days, and that’s it. If you can spend a couple of days in a not so high location like Arequipa (surely worth a visit) you will be altitude-sickness free when you go to Cusco.
Chewing coca leaves does help. Not the tea, but the real leaves. They are not illegal, but part of every day life in the Andes.