It's the way to go if you want a safe experience, although I would say a 25hp shot wouldn't really have chance at causing much damage even at low speeds unless you just held the button for a prolonged time. That is where the controller will step in and save you from yourself!
I have had A Focus sT roll up on me on the freeway. I didn't think much of him at first, until I tried to pull away and the little fucker started pulling ahead. I had to mash the pedal before I began reeling him back in to take him for a walk. If I was stock, he would of easily left me slack jawed. I'm sure that little fucker was juiced, cause I didn't hear the sound of a turbo or supercharger spooling up and those cars are so light that a little goes a long way
I started out with just the SB150i kit and as I began building it out, realized that the controller was essential. You can get by with a push button activated street fighter style system, ala FastnFurious and you will probably be safe with a small shot up to like what you are talking about, so if I were you and I was serious about starting my build, then I would go for the kit first and install it with the 25 hp jet and play with it for a few months until you can raise the scratch for the controller and then add on after that. Believe me...you will want more!
A friend of mine let me drive his classic barracuda, which is extensively modified and when the nitrous hits, it transforms the experience of driving fast! The instant-on power is sooooo addictive that I knew I had to have it. I am installing my first kit ever and can't wait to finally get it done. My install is a direct port, so it is much more involved and will yield alot more power since it is also going into a supercharged engine. Still, I wish that I had gone with a single injector at first just to get a nice feel for it.
Another benefit of having the controller is that you can take it from one car to another. You mentioned you also rolled in a Cooper right? Well, you could do a set up on it as well and use the controller to drive both systems. Must be nice having two rides to play with
You should post a pic of your engine bay. I have seen two configurations of the same Duratec engine, one has the intake near the front bumper, the other has the intake in the rearof the engine bay. which one do you have?