As soon as I saw Due mig I knew the post was from Aus. I bought one 27 years ago and it served me well for 10 years. I welded everything with it including aluminium.
As for the bottles have you seen the guy on ebay where you buy the bottles of him and get them refilled off him as well. They are a company that have depot's all around the place. A large bottle was around the $400 to buy out right full.
As for polarity I would give it a go as is, going by this thread below. I haven't tried it but it seems a lot of guys think there is little or no difference.
http://www.mig-welding.co.uk/forum/showthread.php?t=1339I have always used gas with the Due mig and with the mig CIG Transmig that replaced it.
Last year a neighbor was talking to me and said he bought a new Toolex 160amp gas/gasless mig and hated it and it was crap, and that he should have just bought a ark welder to use up the farm. We got talking and he asked to swap me for my little Chinese ark welder which I couldn't knock back as I only paid $60 for it second hand, and told him this, but he did not want this mig even though he paid around the $600 for it.
I got it and strait up I could see the wire was rusty so I unrolled it till it was clean. the welder welds fine, but I really don't like the way gasless wire welds. It just doesn't seem to get the same penetration and leaves flux on the weld. I also soon found out you don't tack with out a helmet using it as I ended up with a sunburned face. I would say this is because it doesn't have the shielding nozzle when using gasless, and the rays just come strait back at you. I don't even get ray burns like that at 200amps on my gas welder. It also has a lot more fumes from the flux, so if your working in doors it is something to think about.
You will find the expense of the gasless wire to work out the same if not more than running gas if you use it regularly. If you are only going to use it now and then it would be cheaper, but take the wire spool out and wrap it in a plastic bag then put it away in a cupboard.
I will use up the gasless wire here and their out doors, but for any welds that need real strength I will use gas.
Buy a small roll and give it a go and see what you think, but I don't think you will like it after using gas.
Dave.