Pure argon is for TIG welding (almost anything).
And pure argon is also used for MIG welding of aluminum.
Standard shielding gas mix for short circuit transfer mode on steel is C25 (75% argon and 25% CO2). Other shielding gas mixes might be 98-2 (98% argon and 2% O2) for spray transfer mode MIG on steel.
And a brand new Lincoln PowerMig 210 MP has a $200 rebate from Lincoln going on, for a listed price of $999 after rebate. Same price as a Lincoln PowerMig 180 Dual.
Welding cylinders absolutely
DO have to have current hydrotest markings, or have to be restested. Some suppliers handle retests as just part of the cylinder 'exchange', others may charge you for the retest.
Cylinders can be owned, or leased/rented.
As to how to check if a cylinder is a non-returned lease that the seller is trying to 'sell' to you, ask several LWS and see how they handle cylinders. Some only handle 'big' cylinders on a lease basis, others will handle 'big' owner cylinders. Most of the time (at least around here), if you 'own' a cylinder you just end up exchanging it for a full one when 'your' cylinder is empty. You still 'own' a cylinder, just not the same physical cylinder that you may have had originally.
$999 PowerMIG180Dual brand new from LWS
$20 plain Tillman MIG welding gloves
??? tank price depends on size An 80 ft3 cylinder of C25 is $202 at Cyberweld, a 60 ft3 is $153.
http://store.cyberweld.com/migaccessories1.html
??? Unknown mask and 'jacket', plain mask is maybe $25 for a fixed cheapy, jacket could be perfectly adequate cloth $25 welding jacket
$40 retail for an extra 12.5# spool of Lincoln L56 MIG wire, at Lowes or HomeDepot. Less via some sale or fleabay.
~$1100 for 'everything' except the cylinder(s) and the cart. Brand new.
The CL seller is asking way too much IMHO. Pretty much full retail + ~$100 profit is about what he seems to be asking (and I'm being 'generous' to the seller there).
Negotiate way down, or just buy stuff brand new for about the same price or less (and get a jacket and gloves that actually fit you. For all you know, the seller has teeny tiny hands and the gloves are XXS Ladies gloves.

Just fine if that is the size glove that fits you, no good if you need a Men's XXL.)