from what I understand oxy-acetylene welding was used a LONG time ago to replace panels on cars.... and oxy-acetylene would prolly be the absolute cheapest welding set-up (assuming you rent the cylinders)
the downside is that oxy-acetylene welding heats up what you are welding a lot, so you may end up getting bodywork that warps...
but if you want to learn how to weld, start with oxy-acetylene, learn how to manipulate the puddle (or weld-pool) and then move on to something else...
that is of course if you wanna be a good welder
IMO if you start off with mig you won't be doing yourself any favors in the long run...