Re: Tips on lighting, please..
Well besides the MH light you will also need a ballast to run it. When buying a ballast try to get one that runs both MH and HPS. I would also recommend a cool tube if it is in your finances, as well as a fan to extract the hot air the H.I.D. light will generate. As far as running to close to your walls and or ceiling, if you are using a hood with your light you should be O.K.. Check out some cabinet builds and how others have run their exhaust out of their cabinets or tents. If you are not running a hood with your 400 which is not recommended then you may have to take precautions and keep a fair distance(maybe 2 feet or more) from your walls and ceiling. Best advice here is to light her up and monitor it for several hours or a full cycle if you can. Better safe then sorry.
These are my opinions based on information you have provided, I am NOT an electrician, nor qualified home inspector.
ALWAYS BE SAFE AND USE CAUTION!