I've used an ultrasonic type before, controlling with arduino and a HIH4030 Honeywell humidity sensor. The Honeywell is a quality sensor, you can find cheaper, but theirs are used in HVAC system and are accurate and very good quality. Be careful selecting this sensor as there are many very cheap temp humidity sensors out there that are not accurate.
The Ultrasonic generators are the same, try and find one that is a bit more expensive and buy just the sensor not something like was in your link, here's one I'm going to try in the link below, I used a very high end TDK in the past that was over US$100, but I'm not sure that's necessary, won't know till I do a test sometime in the future.http://www.banggood.com/DC-24V-3528mm-U ... ?rmmds=buy
If your going to go to the bother of developing a unit, don't go dirt cheap.