Hey Phil, I've considered this route with my Miller 211... what machine are you going to be using?
I use my MIG on mild steel regularly enough that switching the gas back and forth from C25 to pure argon was going to be too much of a pain, plus all of the reviews for spoolgun success are all over the place. I decided that waiting for a good deal on a proper AC/DC Tig would be the way to go, no nonsense. Every so often Cyberweld has a great deal on demo Miller Diversion 180's (I almost picked one up last year for around $1.3k).
Another cool option if you are up for an entirely new rig... I hear that new ESAB Rebel mulitprocess is killer... AC/DC TIG!!!!!