That's definitely a 400AT, NOT a 420. I would avoid the 400's. They aren't known for holding up well under anything other than light use. In 2007 the 420 was introduced in footshift and ES models, but the automatic was still the old 400AT. The 420's have front disc brakes and are fuel injected. The 400AT's have front drums and are carb'd. The 400's tend to burn up clutches if you do anything other than ride pastures on them.
IRS-independent rear suspension. Much smoother ride, more ground clearance. Better for everything except towing and heavy hauling, in which case the SRA(solid rear axle) is better IMO.
ES-Electric Shift. This is personal preference, but I avoid them. Footshifts are simple, much less likely to have problems. The ES bikes delete the footshifter and instead have an electric motor on the front of the engine that shifts the bike for you, using buttons on the handlebar. In my opinion, it's just something else to break that isn't really any easier to use than the footshift.
DCT-Dual Clutch Transmission. This is the new automatic transmission Honda started using in 2009 on the Ranchers. It has buttons on the handlebar like the ES, but also has an "Auto" mode which shifts for you. If you don't want a footshift, you might as well skip the ES and get the DCT. If you're going to have all that electronic crap on a bike you might as well have the option of auto or ES.
EPS is electric power steering. I've never seen a need for it but most like it.