If you want to read something that is very very different from Modern English read anything written in Old English. The thing to remember about Shakespeare's English is that by the time Shakespeare wrote his plays, English had taken on its modern form. Shakespeare did not write in the Old English of the middle ages.
The English of Shakespeare's plays resembles our "Modern" English. Words were spelled differently, often y's were used instead of i's, and there wll be alot of vocabulary that will be unfamiliar, but the grammar is the same structure as anything written today. The true difficulty with Shakespeare comes from the use of unfamiliar nouns or verbs, a question of vocabulary and not grammar(the structuring of sentences), and this difficulty is usually remedied by notes and annotations in modern editions of Shakespeare's plays.
The sentence structure of Shakespeare is largely identical to modern English. The grammatical structure is that of modern English. The conversations are conversations had by noblemen and commoners alike. The monologues are those of both royalty and commoners alike. It is a human language. The plays are human dramas; the stories of people. The works of the Greek and Roman theater are no different.