1). Onagel perhaps for only one plant, or activated carbon filter with inline fans to clean and then exhaust hot (tent or grow room) air outside for more odor control.
2). Electricity is based on lights your using, and the wattage. HPS lights come in different wattage ballast and bulbs, there is also LED and CFLs which use less electricity but with general rule of thumb yield less for a first time grower or inexperienced grower, although I will admit I've seen some pretty impressive LED and CFL grows that make my first 400w+150w hps grows look silly, lol. Generally you can take the cost of kwh in your state/area, and then times it by the wattage of the light/ballast. If you had 13 cents per kwh, and you used 400w light for 12 hours a day, in 30 days, it would be rough estimate of 0.4kw/h (400w) x 12 (hours per day) x 30 (days) = 144 kw/h. Then you take that 144 kw/h for the month and times by price of electricity, for ex .13 cents per kw/h, 144 x .13 = 18.72$ for 1 month use of 400w at 12 hours for 30 days. This is how I figured it anyway, you can find this out by just googling. Also count for usage of extra fans, air conditioning, or any other additions to control environment for grow, as adding HPS lights add heat to the room that needs to be vented.
3) No clue, leave that to experienced grower to answer.
4) New growers always over baby their plants by nature by majority, I myself included. Care intensive if you are wanting to learn as much as you can about growing, I personally experienced quite a first attempt with numerous problems never making it past seedling stage. Make sure you know what the pH and ppm of the water source your feeding the plant with, the medium (soil) and how much nutrient it has in it already, know your going to need to keep temps out of mid-high 80s for extended periods of times with lights going, so if you plan on going for HPS and quantity to make the grow worth the time and effort, I would recommend investing 110$ on a 5,000btu window a/c to control temperatures if you plan on using 400w+ hps, 150w you might be able to get away without a/c, but you will still need fans and ventilation to bring new air in, and get the old(hot) air out of the grow area.
5) I went into first grow less then a year ago planning on doing 1 plant with 90$ 135w UFO LED light; 6-7 months later and 600$ later, I am using 400w and 150w hps together, synthetic nutes (non organic) cost 50$ for the set, I purchased promix peatmoss soil from ebay as I didn't want to drop a bunch of a huge cube of soil with intentions of small grow. misc items pH meters, ppm meter, pots, all around 10$, tents will run around 80$ish for smaller ones. Lights 80+ for cheapest 150w or cheap ufo LED up to 400w hps cheaper brands for 120$ish. I put about 300 into lights, 50 nutes, 50 misc stuff, bout 50 in soil since i started, 100$ in seeds from seedbank, 35$ onagel odor control. Yeah I'm not including everything here but you get the idea, this is bare minimum stuff, I don't have a good setup at all just using my bedroom with sectioned off part, and I still went 500+ in, but I admit I could have saved 80-90 by not buying LED and just going 150w hps and 400w hps to start with, Kinda a lesson learned type thing.
I decided since I was going to spend the time growing and wasn't limited by funds or situation, I would go for hps and try to get little more practical yield for time spent growing. LEDs are great innovation, can't wait for the cost of the higher quality ones to come down in future, and more powerful ones that come out with better penetration, but with a single plant under a cheap LED I wouldn't expect anything special, the high quality LED lights cost an arm and a leg compared to HPS lights, so its important to know how practical a UFO will be on one plant. Personally I would recommend 150w hps atleast if you wanted to try growing and wanted to keep lights under 100$, but 400w is only 110-120 for ipower brand (cheaper); however you probably would need window a/c if your room is already ambient temp of 75+.
edit: I wanted to be clear, I'm not 100% on the Kwh usage as I don't even pay my electricity its included in the rent, and I had just googled after pondering the same question you had; so if you want to further look into it; you can find it asked many times on this forum or on other forums.