Re: Thinking about buying a mig-welder...
Mig is one of the easiest forms of welding to learn but it still takes practice to be proficient.
My suggestion is to shy away from the cheap big box store brands and go for a name brand. Miller, Lincoln, Hobart comes to mind. The cheaper machines are hard to get parts for and no one will work on them if you have problems after the store warranty runs out.
Practice, practice and more practice is required before tackling something like a bike frame where you're life might depend on your welding talents.
Age and Treachery Will Always Triumph
Over Youth and Skill & "Charlie Don't Ride"