Mountain Biking - Disc Brake conversions

I have an '04 Trek 4500 and would like to get a set of disc brakes for it. The only problem is that my wheels are not disc ready. What would be the easiest way to make it ready, just an axle or a whole new wheel? I'm not wanting to spend a ton of money on these or the brakes. I was thinking about getting some of the shimano brakes and wheels or something. I'm fairly new to the sport so I dont know what all is good and such. Any help would be appreciated. Also, I might sell my bike if I can sell if for the right price. Who makes a good all-around bike for trails and such for under $500. I don't ride a whole whole lot, but want to get into it more. I know Specialized and others make good bikes, but what about Haro? What are the advantages/disadvantages over hardtail vs. full-suspension? Also, one last question, what do you think I could get for my Trek 4500. It's stock with only a few minor stratches, but other than that, its in great shape. Sorry for such a long post. Thanks in advance.

you want the specialized hardrock disc. Its msrp is something like 550 i think, slightly over but if you can squeeze a few more pennies ide go for it. Assuming your fork is disc ready, you can go the more economical way and have just disc in front. That is where you need the most stopping power anyways. For under 100 you can get a machine made front wheel of reasonable quality. THen add another 70 for an avid BB7 and there you are, discs. Replacing only the hub means buying new spokes more likely than not, basically a wheel rebuild which is a another complication and will run between 35-50 in most instances.