Your suggestion will work but it is quite complicated electrically. I need to know if your WI is aquamist system or pump similar to a Shurflo.
If it is an Aquamist system, the link below is a slightly simplied version: but requires a activation circuitry. ( we can noramlly supply it to you free)
http://www.aquamist.co.uk/phpBB2/viewtopic.php?t=401