I wouldn't expect 1.5-2 kilos from a 600 watt HPS, no matter what you do. The 'holy grail' of growing is 1 gram/watt, and to achieve that requires perfection in every aspect and phase of the grow. I'd look for 0.5 grams/watt, and consider anything beyond that a bonus. That way you won't end up being disappointed when you don't achieve the holy grail. So, that'd be more like 300 grams or 1/3 of a kilo, around there, 10-12 ounces per cycle. I have never achieved 1 gram/watt myself, though I am getting closer, and often get around a pound/crop now.

If you keep some plants vegging while you are flowering, then as soon as you're done flowering you can throw already-vegged plants in there and harvest approximately once every two months, and increase your yield/time that way.

There is no set amount of plants, really. You could grow a dozen little plants (flower early after cloning/rooting), or you could grow 2 or 3 bigger plants that you veg longer. If your laws punish you based on the number of plants (if you're caught), then growing just a few large plants might be a better idea than a bunch of little plants...just in case.

Envirolites might seem to save you money, but to get the same amount of lumens, you'd need to use more of them, and they'd then take MORE power than an HPS. Your yield goals are hefty, so I'd highly suggest an HPS: more lumens, and bigger yield, for the power used.

You could build a cabinet rather than dedicating an entire room too. Cabinets do have some advantages over rooms: they're portable (if you put casters on the bottom), you can saw and cut them all to hell and it does no harm to your home, stealth, and so on. I love cabinets myself, and have zero regrets going that route. Mine is also in my attic, which has a sloping ceiling.

Here's a pic of my flowering cabinet, with three plants in it, and another pic of the same cab after harvest (I use that cab for drying too). Those plants were vegged too long and grew all out of control during flowering. Anyway, cabinets are a decent alternative to dedicating an entire room, so figured I'd mention it in case you hadn't considered that possibility. Somewhat stealthier too; I've had people in the same room, walk right by that cabinet, and never suspect a thing.