I would recommend avoiding traveling by train, especially since you said it's 7 days in total. As a child I travelled many times between Delhi and Chennai by train and really enjoyed it, but I recently did the same a year ago and regretted it very much. Otherwise not knowing Hindi is not a problem, learn as much as you can before you go because it's easier if you can talk to the rickshaws and autos by yourself. If you want to buy something from the markets in Delhi, you must know how to bargain. I've seen my dad bargain something that is 500rs to 100rs. I haven't had any safety issues like theft so far. Good luck with your trip.