There are many nutrient/pH injection systems on the market, though their not exactly cheap. At least not in my book, but I guess "cheap" is a relative term and it all depends on how elaborate you need it to be and how deep your pockets are. I know you said you wanted to build your own system, but I don't think you mean you want to design the parts on a cad program and have them manufactured. I assume you mean buy the parts and put it together yourself.
I don't know what your motivation for having an injection system is to automate nutrients and pH, or the type of hydroponic system you want to use it with, or even how many plants you will be feeding with it. So I have no idea whether doing so makes any sense. You can buy the parts separately and custom design it for your needs, but they still cost money.
It can range from a 3 to 4 hundred dollars for one nutrient injector to $5,000 for a multiple nutrient injector system. pH injection increases cost since it needs to be commuter controlled and requires both pH up and down injectors. If the hydro system recirculates the water, you need to add sensors and computer controls to the nutrient injection system as well. The point is unless you just have fun spending money and find it challenging and rewarding, there should be a economical reason to go to the trouble and expense, otherwise your just wasting money.
For some designing things and the challenge of figuring out puzzles and overcoming obstetrical s where all the fun is. That's what the food computers are all about, But just because you can make something more complicated and expensive, doesn't mean it's better. However if your growing a large number of plants for profit, then it makes economical sense.