Tip #5: Note that this is the only tip where I will mention functionality, and it is this:
Make sure how exactly the wireless motion sensor alarm system notifies or alerts you when movement is detected? Is it a recorded voice? A siren? A beeping sound? A flashing light on a panel? - This is critically important because if the information about what is going on, is not communicated clearly, it does not help you at all...
There. You can now consider yourself educated on the finer details of buying these systems.
Go forth and be safe!
Full disclosure: We sell the Drongo wireless motion sensor alarm system, and if asked I will obviously tell you that it is the best thing in the market, but selling Drongo stuff is not what this article is about. I am giving you broad advice, and if that leads you to buy a competitor's product, I will be happy that you have done so after receiving excellent advice!