Substrate ingestion is not a big deal. The worst that could happen is a stuck piece in the mouth which you can just remove with tongs. Even that doesn't happen all that often. It doesn't matter if they eat the substrate, their digestive tract can handle it.

This is a nice cheap digital thermostat that has been tested out by one of our members here: http://www.amazon.com/Hydrofarm-MTPR...1529683&sr=1-7

So far, it works very well with UTHs and flexwatt and the price is very good for a digital thermostat.