The people who would have just the information you need are likely to be the manufacturers of commercially available systems. While you could reverse engineer to get some of the answers it is unlikely you'll get them to share all of it because it is the result of a lot of hard work/expense and what might give them a competitive advantage.
Reverse engineering refers to having a completed product on hand (or at least the plans for it) and looking in to it as a way of gaining insight to the kinds of things you've asked about. You might obtain sales or O/M manuals on various products to see if there is a common theme. Be careful though, the choice of motor, for example, may have more to do with what is the best choice for the manufacturer rather than the best choice for the application. An example - if ABC Parking Systems is owned by XYZ International - and XYZ owns 123 Motor Manufacturing then the choice might be driven by that more than by what is best for the application. Still, a review of available commercial products might be useful research.
If the information you've presented represents all that you have it is not enough for someone to even begin to answer the questions. What does the gate look like, how many cars, how will cars be detected, how many in/out lanes, etc ,etc.
If you are really doing this for real it is possible that time is money. If that is the case it would seem unlikely (though possible) that you could design and construct a reliable parking control system for less money than someone who has done it already. There may be other benefits to what you are doing - if only learning - so in your case, the project might make perfect sense.
Best of luck to you.