It sounds like it can be done. First you need to find out what the tach needs for an input, what levels, speed and form. Then you can have the arduino output that to the tach. After you get that working then start on the control.
I'd start by going out on the web and seeing what is already out there, no need (or market) for something someone else is doing already. Then put together a list of what you want it to do and how you want it to do that. Once you have a plan start looking for parts that will do the things on your list. Too many people start with the micro and then ask how do I do this with this part and sometimes the answer is "you don't"