You usually rent the bottle from BOC, and pay per refill. Its about $120 a year for the bottle hire (paid quarterly), and around $75 for a refill of argoshield light (normal MIG gas). You can also try Linde, Air Liquide and SupaGas for a price comparison but they're all pretty close. You'll probably need to buy a regulator as well, about $120 for a decent one.
120 amp is a little light on but should do panels and light fab work up to about 4mm thick steel, so its enough for most car work - just.