Wow, this is my first post since registering!
A few years ago, while working my way through college, I was the PR officer and a CAD tech for a local construction firm. I had the opportunity to fiddle with many different programs and most all of them came up short of the king of all CAD programs, AutoCad. I know it's major bucks, but you might be able to find an older version used for not a ton of money...on Ebay, AutoCad R12 is going for just over $100-$150, depending on what packages it comes with. Depending on how much you want to spend, there's R11, R12, R13, R14 and 2000, 2002...
I would think R12 would be about as old as you'd want to go.
If you are only going to be doing 2D drawings, you can get by with AutoCad LT, which is MUCH cheaper.
I wouldn't play around too much with some of other software out there as most places you might submit a file to are most likley standardized on AutoCad.
Hope this helps just a bit.
-Danny