Having the tank next to a window if the temps drop that much could work I think. I keep a few small terrariums on my windowsill and they drop to about the outside temperature. And Paul is right. You NEED airflow. It sounds to me like your last truncata probably died from rot. Usually hard water will not kill nepenthes, but it WILL break down the media that it's in, and that will create bacteria loving conditions and your roots will rot, causing the whole plant to turn brown like you said before. I have a setup very similar to the one your describing and if I didn't have a fan blowing on my plants, they would surely rot in a short period of time. Especially if they're wet. And the water at the bottom of the tank that Drew is talking about, is good to keep the humidity, but you DON'T want your nepenthes to be sitting in it all the time. I think the few basic things you need to remember is that:
* love moisture, although they hate to have waterlogged media. It will kill them faster than anything else in my experience. The sundews need to be kept sitting in water, but nepenthes will almost always rot that way (I say almost because there are people who grow them using the "tray method").
* highlanders NEED the drop in temps to grow well
* even ONE nepenthes will outgrow a 10 gallon in a few months. I started with a 20gal 9 months ago and I now have (literally) a porch full, two indoor terrariums (one 20 gal and one 40gal) and another indoor grow shelf that is bursting with plants. Even if you trim them back, they will very soon outgrow your setup and then you will have spent all of this money for nothing. With your budget I think the best thing to do would be to build a grow shelf like MATO has. You can check out his thread under the "greenhouse and terrarium" section on this forum. It's pretty much a grow rack that's been inclosed by reflective coated insulation (I got a huge 10ft by 4.5ft piece of it for $6!). There are T5 lights (if I remember correctly) on the top and an ultrasonic humidifier that blown into the area with a small computer fan to push the mist around and keep the air moving (essential!!!). If you can't do that, then at LEAST get a 55 gallon aquarium. If you truly only have room for a 10gal then you really don't have room to grow nepenthes. Although depending on where you live you can grow them outdoors or on a windowsill very happily. I hope you're able to put together something nice!