Does anyone have a Module just to check if SNMP is working at all?

Yeap, I've also implemented snmp and wmi checks like that in the past. While there are some standard DataSources that do alert on NoData, many do not. It's easy for snmp to break and no one notice. I think I've implemented something like what you have above, except as an instance that only appears when there is a problem. That is just so it doesn't always show up resource tree for everything and I don't need to track/graph that data. But that also means there is a 15min delay in alerting but that was acceptable.