Don't let him scare you off, its as easy as you want to make it. Just stay away from the usual first time probs like over fertilizing, and take your time and yes you can save some money by going to wally world, but if you want the best chances for success go with a fert formulated for what your growing and a good soil (ocean forest or fox farm) if thats what you're using from a hydro shop.
I'd go with the 400watt because like spliff said you'll likely grow more next time and a 400w light isn't significantly more expensive than a 250, but for 2 plants 600 is overkill.
Good luck,
NH