A large PSU will not cause any problems, but go for build quality over the number on the box. Seasonic is very good.
Again, at that price range you can afford smart and high quality components. A 500GB SSD like the Samsung 840 evo or Crucial M500 would make the entire system feel more responsive, and they're only about $300.
Spending more than $200, or even over $150, for a case ought to be an indicator that you are buying a specific case for a specific purpose because only that case can do what you need. If that isn't the case, look into the corsair brand cases. From nice mid-towers under $100 to huge towers designed for hardcore water cooling, corsair cases hit a very wide range of prices and features.