Humidistat Control for Exhaust Fan in Crawl Space
That is a bold request by the buyer. He has you doing his future projects before he moves in!
The thing is, when the humidity is high enough to kick the fan on, it is usually caused by high humidity outside that drifted in. So the fan will just be blowing the same humid air thru the crawlspace, causing next to no reduction in humidity!
I have a fan, and they are nice. It has a timer, but I manually turn it on only when the humidity drops outside, like the day after rain, which definitely helps. I monitor the dewpoint outside.