If your present wire welder won't weld sheetmetal without burning holes, there may be something wrong with the welder or your methods/techniques.. examine both closely. You don't say if you've actually tried using the Century model, or if you're just comparing advertising numbers.
You may not need a new liner in the gun/torch assembly to run .023" or .025" wire. The C25 argon/CO2 mix is more expensive, but most likely the best choice for sheetmetal and small wire. Small wire size, low power output and C25 gas is what will make sheetmetal work very easy.
When repairing rusty sheetmetal, any rusty spots are best avoided. The weld should be applied where the surface is absolutely clean steel and free of rust, and where rust hasn't eaten away at the back side of the body metal. If the body metal is rusted badly on the back side, the patch area needs to be enlarged/extended. The skills for proper welding are in the user, not the machine.
Don't get too carried away with the model numbers and marketing hype.. even the better brand name 120VAC wire welders are just 90 Amp units. Ernie L (SEJW group) and others will confirm this. The possibility of a little more output amperage is realistic only if the unit requires a 30A 120VAC supply circuit. Another aspect that many get carried away with is duty cycle.. unless you're making long, continuous welds at high output current, duty cycle isn't paramount. During most home shop welding, the user is going to pause, changing position or other small interruptions that reduce the actual weld-duty time. I would consider a better warranty period of more significance than a slightly higher duty cycle, only because new stuff sometimes fails. A good warranty and parts supply chain outweighs a full power, 100% duty cycle IMO.
There are numerous quality 120V units available, that have parts readily available in most locations. Hobart is one very good brand, with wide availability from what I've seen, but I haven't needed warranty repairs or major parts replacement. All of the genuine Hobart consumable parts are readily available to me locally, and if I would need a gun assembly or new liner, they're readily available online, or thru local dealers.
This Mm 211 unit is multi-voltage unit with extra features like the Auto-set and Smooth-start which appear to be additional electronic circuits that may be nice, until they fail. I don't know what spatter-free start is, but it sounds like marketing crap. More features are generally just more things that can go wrong, and most likely need to be returned to a service center for repair. The smaller Millermatic wire units also have the Auto-set feature.
A good quality 120V unit should be economical to own and use with consistent performance evey time it's turned on.
Since you already have a gas regulator kit, you might consider a basic model quality wire welder that has the solenoid valve already installed (many do).
The Hobart units still appear to be made in U.S.A. and they're availability is as widespread as any other manufacturers. You may not find a continuously adjustable heat range on some models.
Years ago, my job involved setting up a small fabricating and machine shop, and to produce small machines for the employer. The wire welder welder that was bought was a cheap imported bottom-of-the-barrel quality unit, that required more time repairing and adjusting, than actual welding.
Later, for my own use, I bought a Hobart Handler 135, and after years of occassional use, have never had any problems with it. It still works just like it did when it was new.