a 6013 is just a type of welding rod. Reading into it before posting this it appears 6013 seems about the most common type used but here is the speal if your interested

Different Types of AC Welding Rods | eHow

no doubt this clever lot on here will tell you these are the wrong ones... but they are the ones ive always used. Work perfectly fine all weather for me

regarding your design...

Good start... ive seen a lot worse.. but in my opinion...

*id have a center leg on each side rather than use the bed horizontal as brace (you could use that aswell)
*more bed frame crossmembers
*forget the single ballscrew arrangement for the X, use two one on either side
*triangulate the corners wherever possible
*If you think its over-engineered at any point... its not.. you watch it dance round the workshop like its made out of carboard when its cutting. You wont regret stregth in design

read ALL the other build logs...

My machine

http://www.mycncuk.com/forums/router...ation-cnc.html

is similar to your design