Welcome!
Assuming you'll be the captain, read this first to get your tandem experience off on the right foot:
http://www.gtgtandems.com/tech/propmethod.html
For bikes...as others have said, used is the way to go if you can find what you want. Santana, Co-Motion, Cannondale, Trek are all brands that appear with some regularity on craigslist in our area (SF Bay, not sure where you are at). The challenge is waiting for something that works for you. If you can stay ~1999 or newer you should get a decent bike that doesn't need too much in upgrades. You didn't mention if you are looking for road or mtb, but there are choices for both.
The KHS is the cheapest of the "serious" tandems. Reviews I've read have been fairly positive -- the main knocks being that they aren't as comfortable on longer rides, and the component package is lower end. Cannondale is roundly considered to be the best bang-for-the-buck. If you end up enjoying the tandem life you could outgrow the KHS pretty quickly, while the Cannondale has the potential to be the last tandem you need for a long time. And it has a frame worthy of upgrading components on. There is another thread right now discussing the availability of new Cannondale tandems. You might want to give that a read.
Santana and Co-Motion are the big names in performance tandems and they have a wide range of models. There are numerous other smaller manufacturers (Rodriguez/R&E, DaVinci, Calfee, etc), and
www.tandemseast.com has a house brand (Hokitika) with a made-in-USA chromoly frame that looks like a great deal, although I haven't heard much about them.
Anyway, go take a look at craigslist. When we decided to take the tandem plunge (last November) I found the bike we ended up buying the first day I looked. Sometimes you just get lucky!
Joe