Your selection of the M&P .40 is a good choice....the advice on a 9mm Pro is also well thought out on the expense side of things. The 9mm Pro is highly sought after in the IDPA/USPSA matches.
If possible, try to live fire one one of each and make a decision from that experience.
In the competitive world it would behoove you read the rule books of what ever discipline you find interesting or available in your area. The rule book will guide you to what things are legal and not. Sometimes what is legal in one is not always legal in another....
I would also find a local match and check out what other folks seem to be using and observe.....many will offer the use of some of their gear to lure you into the sport.....(done that many times myself to encourage new recruits)
This can also be a great place to see if anyone has what you are looking at....most always they are more than happy to let you live fire it.
Most of the time you will need at a minimum a holster that covers the trigger guard (USPSA)or is an IWB design (IDPA). a sturdy belt, at least 3 magazines/speedloaders and pouches to keep them in. (most have 5-6 mags/speedloaders for their gun), a range bag of some sort to carry your gear in, eye and ear protection, a concealment garmet (IDPA only), approx. 200 rounds of proven ammo, proper clothing as per the elements, a willingness to learn and to pitch in with taping targets, picking up brass etc. have fun!!!
You will find everyone will be quite willing to give you a hand on understanding the course of fire, that you are safe in doing so and that you will never have to be first on the course.
Never be afraid to ask anyone a question....we were all new at this once as well and had the same questions as you.
In several clubs that I have been a member of they would sort of assign an experienced shooter to accompany a new recruit and mentor them. They might so the same.....
Did I mention to have fun????
Feel free to ask on this forum as there are many of us that are competitors/Instructors/and all around good guys......
Randy