Ary,
Check out this site.
http://www.mylincolnelectric.com/Catalog/consumableseries.asp?browse=104|2030|
I run 7018 all day long for carbon steel pipe. Personally I cant tell to much of a difference in running it on ac or dc, except for a little more spatter. 6010 and 7018 are the two most commonly used rods in the pipe fitting industry. I have been saying for a while now that stick welding is a dying trade. All the pipe we put together lately has been heli-arced (tig). Anything from carbon steel to titanium, the only stick welding that is done anymore is low pressure boilers and chiller systems and pipelines. But enough of that, I would go with the lincoln tombstone welder and try to pick it up used, just make sure you run a few rods in the different heat settings. Also if anyone is to go out and buy some 7018, get the newer excalibur series, same price if not cheaper than the std. 7018, but runs 100x better.
Good Luck