Lining the walls with mylar may be a bad idea. If moisture accumulates back there you'll have mildew or house mold problems in no time flat. Better would be to get a vapor barrier/fungicidal paint like Killz in glossy white. It's not quite as reflective, but the difference between it and high-quality mylar is negligible. If you really think you aren't getting the most out of your lights, build an enclosure for the plants or a parabolic reflector for the lights. But one way or another, you want air along the walls to be freely circulating. Even if you don't give a damn about what happens to your house, wall mold will be bad news for your plants.
To hang lights from the ceiling at a substantial distance, you'll need to go with a minimum of high output fluorescents - T5HOs are probably the most accessible, although there are high output and very high output tubes/fixtures available in other sizes and form factors. I got mine from
http://www.contractorlighting.com/ and am fairly happy with them, especially given the price. High-intensity discharge lamps, like those used for commercial horticultural applications, might be better for lighting a whole room from the ceiling - T5HOs seem to work best two to three feet above the canopy. Information on HID vs fluorescent lighting online is rife with non-expert opinions by, erm... a certain industry which is... less than scientific or credible. But a discussion of HID lighting as it pertains to Nep culture can be found here:
http://www.********.com/nepenthesuniversity.htm
(Whoops, banned URL I guess. If you're curious you can probably figure it out with Google.)
The tradeoff is such: HID lamps (sodium vapor and mercury halide) run hot, and absolutely require some sort of ventilation and/or cooling in an enclosed space. If you want to be able to leave for extended periods of time without worrying about equipment failure, you'll need to invest in two or three of everything for redundancy and to have replacement parts on hand. The upside is that in the long run they're a bit more energy efficient than any fluorescent technology presently available. If you wish to fill your whole room with plants, plan to stay in the hobby long-term, and can afford the initial equipment investment, they're undeniably the way to go.
Fluorescents simplify the equipment needed for a viable setup, scale better for small to mid-size setups, and boast a much more affordable entry cost and lower risk of catastrophic failure. High-output fluorescents do get hot, and you'll probably still want to provide some air circulation and temperature control. But, they don't get so hot that they'll fry your plants in less than a day if a fan fails and you aren't around to catch it. You can jury-rig your ventilation out of cheap, non-professional-grade parts and ventilation is not so critical that you need a backup system or spare parts on hand. Fluorescents also offer a greater degree of flexibility in terms of spectra available, which may mean more to you if you're interested in experimenting with different qualities of light. But the biggest advantage and distinction is probably entry cost; a basic fluorescent setup will run you 10%-25% of a basic HID setup. You'll pay slightly more in power and maintenance over time, but the difference is not so vast that you couldn't justify starting with fluorescents and upgrading to HID later on when you know you're in it for the long haul.
If you want to go
really cheap, you can use plain old 40W, four-foot T12 fluorescents. If you have a few smaller plants, it will be much more cost efficient, and is a perfectly viable choice for many growers starting out. But once you're lighting more than, say, nine square feet, or your plants grow in excess of two feet tall, you'll make your life easier by upgrading to some sort of high-performance lighting. I recommend against trying to use anything smaller or at lower power. Your present lights
might be OK, but we need more details about them to know for sure, and you'll need to keep them very close to your plants - within a foot, and preferably at four to eight inches. From the sound of it though, you have nothing to lose by at least investing in T5HOs unless money is very tight.
As for rainwater, sounds like yours should work fine, but one hint I picked up from a person who uses rain barrels is to always let the first few hours of the first recent rain wash down the drain rather than collecting it. You roof is almost certain to be free of hazardous chemicals after this much time, but accumulated organic matter like pollen and leaf litter adds substantially to the nutrient content of water and can help grow nasty things you don't want. Nepenthes are even more tolerant than Sarracenia in terms of water quality, but having really clean water is still desirable if only to give greater control over what your plants are getting - if you really want them to have something to eat in their water, you can always fertilize.
Also, total side note, but from what I've read and heard from Nep authorities, even lowlanders appreciate a slight nighttime temperature drop. I used to think constant temperatures were best, but I have it from a number of reliable sources that a change somewhere around 5-8F is tolerated or even preferable. While we're on the topic of air quality, saturation is not crucial - and may even create problems - but you need to be able to keep the minimum RH above 50% or so to prevent difficulties. Possibly more for lowlanders; I grow mostly highlanders because they're much easier in my neck of the woods.
Hope that helps.
~Joe