I would go with the 400 as well and have it setup in a square box style rather than lined up!!
Only problem I have with all your questions is you're not doing the research and are going to get in so deep and realize you need more items than your pocket can cover. Make sure you fully understand that you will also need an extraction fan, 1 or 2 fans for internal air flow (depending on area n fan size) a carbon filter/scrubber, nutes and there will be other odds n ends as you grow along!! Make and plan your list before buying if your on a tight budget. Running low on cash, Visit the DiY section and build your own scrubber to help save on some coin. Remember your just starting out and need the basics, after that u can upgrade when extra coin is available..
RIU hands down has more info than any other site i've been on. A lot of knowledgeable peeps in here!!! Peace