I've read that the 110 EVSE that came with the car needs some interior soldering to work in 220. Then I've read that it's a plug and play as long as you have a pigtail adapter cord. Which is correct? I'd hate to plug my EVSE into a 220 outlet and end up frying the EVSE or the car.
If you have the EVSE that came with a 2016 or newer Volt, it's just a pigtail that is needed. If you have a first gen (2011-2015) Some can be modified with internal soldering, others are 110 only. I believe the 2013+ require internal modification with the 11/12's being 110 only.
As for options. Pretty much anything out there that your budget can handle will work fine for the Volt. If you see yourself upgrading to another EV or EREV in the future that has a higher rated charger than the Volts 3.3/3.6kW onboard charger then you might consider a 30 or 40 amp unit. If not, anything that's in the 20 amp range will be suitable for full speed charging.
One side note regarding the use of a pigtail on the stock EVSE for the Gen 2 volt is that it will be limited to 12 amps. It's not a significant decrease, but will require about an additional 30-45 minutes to charge when compared to the full 16 amps that a traditional EVSE can offer. If you're mainly doing overnight charging, this won't impact you since you'll get a full charge in about 5 hours from a completely depleted battery.
If you wish to pick up a pretty cheap EVSE, there are a few threads on the Duosida EVSE which can be had for under $200. The nice thing about that unit is that it's also a dual voltage depending on which style plug is installed (no modifications needed, just plug swap) and will allows a full 16 amp output at 220V. ChrisTX did a review of sorts on it and his conclusion suggested that it's a reasonably well built unit. On a side note, he also sells an adapter pigtail to use 220 on your existing 2016+ EVSE.