One light for ten plants is a stretch, but as in most aspects of growing thats a statment that needs clarification. You can grow your plants small and flower them in three gallon pots under a 1k hps fixture. But most people like to grow the plants a little larger and flower in at least five gallon containers if theyre not running a sog.
A 1k watt hps can cover an area 5x5 but the hood is important when trying to dial in the coverage area. Most, if not all of the leds cant do that. If you want to run 10 plants under a single 1k hps you may want to look into a light mover. A 1k fixture runs 6 amps. Check how many availible amps you have going to the room on the breaker panel. You can only use 75% of what's available safely. You can run extention cords from other rooms or circuits through the attic and drop them in if you have to.
Co2 is necessary for a sealed room but if you want to run co2 its best to get a monitor and a control unit and burner. Thats expensive. Being new to growing i would recommend just using an air exchange fan which would just vent the co2 out of the room. And if you run sealed room, air conditioning becomes a problem more than with fresh air coming in. With fresh air intake you can run your lights at night and be pulling air in from outside where its cooler at night so its like free ac for the room. Running lights during the day time can be a problem if the outside air temps are in the 90s or higher. You will need to run ducting if you are running a carbon filter with air cooled lights.
Rain water is fine to use on the plants if the ph is good on it.
The biggest problem most people have when they dive into growing is keeping the room at the right temps and humidity. Having the right ac and dehus installed is crucial to healthy plants. Its often overlooked because its very expensive but it will make or break a grow. Cold plants dont grow, and hot plants stretch and nodes space out all over usually. Plants with too much humidity mold or get powdery mildew and those things wreck harvests. Weed is easy to grow, it grows like a weed. Keeping the room in check is not easy but if you can your golden.