Does the ADC-V520IR need PoE or PoE+
Neither. Alarm.com wireless cameras are not POE devices. 12V barrel connector and included power supply.
Are there any cameras we can use with the system that will work with poe?
Yes, if you are looking specifically to use POE the ADC-V720 Outdoor bullet and the ADC-V820 Indoor dome are models supporting POE.
Do you know if the V720 was good? I see I can find them on ebay, but how was it? What features or how is the v721 better?
The ADC-V721W does not support POE. (There is no ADC-V721 non-wireless version)
From a specifications standpoint, the two are largely interchangeable. The V720 has a larger field of view. Data sheets: V720. V721W.
If you are looking for wired POE, the V720 is your best bet. The tail of the V720 has a male RJ45 connector, so a coupler is typically used. You can run the tail directly into the home interior (typically attic space) or run your Cat cable outside and mount a junction box to keep moisture out.