Jonathan is right, however all normal mild steel wire is coated (usually copper), otherwise it would just rust.

The difference is gasless wire is flux cored, and is basically the same as arc welding, without the need to stop and change rods. The main drawback with gasless is cleanup, and the wire is more expensive. On the flip side, gas has no cleanup, wire is cheaper, however gas is expensive, especially if you have to resort to disposable bottles.

You should be able to pick up a mig welder for £120 secondhand, but you'll probably struggle to get one new for that much. The main thing you need to know is what thickness of metal you're planning on welding. 90A will struggle to do anything above 3-4mm, and 120A reaches it limit around 6-7mm. You can weld thicker, however it involves spray transfer, which gasless won't do, and it really abuses torches (I have been known to set torches on fire in the past!), and works the welder hard.

Mig welders are not exactly complicated machines, and the only main drawback with cheaper ones is lack of duty cycle, which if you're only using occasionally isn't a major issue, however you need to make sure it has enough amps to weld what you want it to.



Personally, if you're not planning on doing any thin work (i.e. below 1-2mm), I'd consider an ARC Inverter, as the DC supply makes stick welding far easier. My spontaneous ebay buy of a DC TIG inverter is one of the best things I done. I originally bought it to try TIG welding, however once I realised how easy it made arc welding, the old buzz box hasn't moved for years, and I've done jobs I wouldn't of touched otherwise (last one was a JCB bucket repair which needed low hydrogen rods due to them being made of sprung steel). Mine isn't a cheap one though, however that's due to all the TIG options it's got (High Frequency, adjustable ramp times, post gas time, current switching on button presses) and they're not used for arc welding as you only need a current setting.