Introduction:tense correction

Hi, Please correct this. I am particularly troubled by the choice of using the present and present perfect tenses in situations like this. Please tell me why you would use the past, rather than the present perfect.

Brief biography:

John Doe has graduated (graduated??) from XXX University in the U.S. with the Bachelor of Art degree in honors, and a Master of Art degree from the same institution. Before coming to XXX, he has worked (worked??) as site coordinator for international conferences as a government employee in XXX. Prior to that, he was involved in teaching science in many high schools in the U.S. HIs current hobbies include cooking and jogging with his three-old Chihuhua, which he bought when he was entralled by the grand, loud sounds it makes when it beseeches for good dog food.

Also, I am somewhat lost as to what is the guideline on using just the adjective to denote a noun that has the definte article 'the' in front of it, like the below. I think I have seen more than enough adjectives being used to refer to nouns that are preceded by words like 'best', 'expensive', 'good', etc. I understand you can write 'the best' to denote 'the best people'.

John Doe purchased the most expensive pizza in town for XXX dollars. He remind us that when you want the best and the expensive, you must shell out the money or you won't able to taste the quality the best and expensive have to offer.