Can one eat a balanced diet if one only cooks once a week?
The types of foods that reheat well are also quite suitable for being balanced in one main dish. Generally speaking, dishes with stuff in sauce freeze/chill and reheat well. So stew, curry, chilli, ratatouille etc. should work. Any of those can be made with plenty of veg, which is important if you're aiming for a balanced diet. An accompanying carbohydrate side can often be cooked in a similar time to reheating the main dish, so this doesn't need to be included (unless you want to of course). From the point of view of sticking to nutritional guidelines you can do much better this way than buying ready prepared stuff, and cheaper. 

Freezing in single portions is important; I tend to use containers intended for takeaway meals.  Freezing means after 2 or 3 sessions of cooking you've got a variety of meals.  I tend to defrost in the fridge, allowing 24 hours.

Personally I batch cook about 5-6 portions most Sundays, but don't rely on reheating these every night. Quick simple meals (like a stir fry or an omelette) add a nice variety.