Cheaper: DIY

If he knows electronics then the programming is a piece of cake. I use an Arduino with a DHT22 temp/RelHum sensor for my bread proofing box.

To use it for something else is no problem. Most of the code is copy and paste (for the DHT22 there is a ready to use library). For LCD there is a library too. Not sure about the Ph sensor.

If you go that way I would start with planning the hardware around it, sensors, pumps etc.

There is a brand called Atlas Scientific (http://atlas-scientific.com/) that makes reasonably priced sensors and also seems to have some software/arduino support and designs. Not sure if it would be as simple as plug and play, but it looks like a good place to start. I'm sure if it came down to it you could probably talk to the company themselves and be networked to someone who has the code already for each sensor. I work with a multitude of their equipment and really find the quality and accuracy to be excellent.