If all you will be working on is cars, then the SP135 plus or 175 plus from lincoln is all you need. (actually, the 130 to 180 series from any of the top 2 or 3 manufacturers will work)
Kid,
I'm a little slow at the moment so please have some patience and help me get things clear... you have something like a large single phase [there's an oxymoron huh?] GTAW welder in your shop now, and want to supplement it with a GMAW for automotive work up to perhaps 1/4 inch steel... quarter inch steel seems pretty thick next to "gauge material" on fenders &c...
...so my attention gets diverted from the arc characteristics to the wire feed mechanism as I consider how many different sizes of wire I'll be using, how much effort it takes to replace and adjust the drive rollers while I change the wire, and if I can use the same (sorry I can't think of the name of the conduit the filler wire feeds through) liner tube with the smallest wire as for the heaviest wire I would like to run and just verify my suspicion that "it doesn't matter"...
...but it might be easier just to run the TIG for a short bead or three; if all that potential bother is real you might be still setting up the MIG while the TIG weld would have been already cooling off.
If required to push 035 or heavier wire, other machines may claim the ability, but you get versitility from a 250A or larger power supply on those larger wires which a 150 ro 175 cannot match... I'm not sure of my precision here, but those last 5/32 of an inch of so of capacity probably cost about a thousand bucks extra.
I'd respect the experience of a guy who owns one more than my own worries, and I'm a little curious as to how my sensings on the matter stand up to practical reality, but I believe the concerns should be considered whatever size MIG you choose.
Regards
d