Venting the room is important but not nearly as important as air cooling your lights. What are you planning on running as far as light? If it was me i would do 2 1000 watt lights, air cooled with 8 inch ducting, and an 8 inch fan.

The room only needs enough ventilation to replace all the air in the room every 3 to 4 minutes. Figure out how many cubic feet your room is and get the appropriate fan. For example, in a 400 cubic foot room, a fan that does 200cfm would clear the room every 2 minutes, which is more than adequate. If you need an active intake just match it to your exhaust, if anything slightly less powerfull intake than exhaust. You can use one fan to exhaust both rooms, just duct it into one fan, they make Y's, just make sure and do your math first. I run my exhaust and intake all day and night, but turn the fan off that cools the lights 30 mins after lights out.

panda film works just as good as mylar. If you dont get the mylar perfectly straight and flat its really not any better, and probably more expensive.

Soiless mix is ok, you will be happy with it, but DWC is the way to go, bigger yield, faster growth, and its easy, low maintenance.

I would do a drip system on a timer, make everything as automated as possible. That way you cant forget to do it, and it keeps everything consistent, less variables. The environmental controls are very nice, but you can accomplish the same thing with basic timers.

Make yourself a bubble cloner, there's lots of good directions on this site for that. Maybe someone else can speak up about seedlings? I just deal with clones. Pretty sure you can pop em and put them right in your soilless mix

C02 will increase your yield by alot, but alot of dutch growers say it degrades quality, because it pushes the plant to hard. Experiment and see what you prefer.

Good Luck!