What should I know before purchasing a sword?

Since so many people find swords "cool," you are very likely to overpay no matter what. If you just want to find a replica of a historical sword, they are relatively easy to find. The sword you will be buying will almost certainly be unusable as a weapon, but you probably aren't (or shouldn't) be buying a functional weapon in the first place. If you are looking for a "real" historical weapon, be prepared to pay a lot, for something that may very well look shabby rather than "cool." Most swords were tools, not works of art. An attractive, functional, and historical sword will set you back far more than you had probably thought of paying. Another route would be to purchase a modern "weapon" grade replica of a historical weapon, which will also cost hundreds.

what will you be using the sword for? how durable?

Don't poke you eye out.

To quote G.R.R. Martin, "Stick them with the pointy end".
I'd suggest to just search eBay. This is the first thing I found, and it looks quite cool: http://cgi.ebay.com/FUNCTIONAL-HAND-FORGED-ZATOICHI-SHIRASAYA-KATANA-SWORD_W0QQitemZ330150747686QQihZ014QQcategoryZ43338QQcmdZViewItem
But I'd suggest you look for more.

The story behind it would be the whole history of katanas and the technology of making them.

Here's another thing I just found, by just searching for "sword" on Squidoo:
http://www.squidoo.com/SwordsOnline/
Seems quite educational at a glance. Hope that helps.

Check out antique stores. New Orleans Louisiana has some antique stores that have some cool ones. So wherever you live you probably could find something you will like. Remember get the one you like dont try to impress others.

When you see it and the only word that comes to your mind is "bitchin," then you will have found your sword.

Well, it honestly depends on if you plan on using it to thrust or to slice

IT really depends on what you will be using this sword for. wha you do need to consider with purchasng a sword is its tang... how much the blade is in the actual hilt of the sword.depending on the type of sword your looking for the tang is an important factor to its durability.

yeah, i appreciate all the links and information (i definitely didn't know the relevance of tang and so didn't think to look it up). i am a martial artist, so while i intend to display the sword most of the time, i also plan to use it to practice forms and perhaps cut straw mats, etc. so it won't receive a terrible beating, but i want it to be as close to authentic without being super expensive. thanks again, everyone.