I have a system using NRF24L01 radios and Arduino Nanos. If you're willing to wait, all you require (5 radios and 5 Arduinos) can be had for perhaps less than $30. PIRs on ebay are also inexpensive. Don't forget power supplies; other minor components will be needed before you're done.
The receiver in my system is in the basement and the farthest transmitter is on the second floor. Conditions in any environment will determine whether these radios, or any, will reliably work.
Bear in mind that most versions of NRF24L01s are not breadboard friendly, meaning you will need adapters. My 'adapters' are ribbon cables I made that are NRF24L01 friendly on one end and standard breadboard spacing on the other end.
Another consideration is that breadboards are not intended for a permanent installation.
Once a 'detection' is received by the receiver, what are you intending the receiver to do?
For NRF24L01 radios, there is much information, such as forums, data sheets, Instructables, and Youtube videos available. Even so, being totally "new to Arduino", this project (as a whole) may be overwhelming and might become no fun at all. However, breaking this into separate projects, i.e. Aruino/PIR and Arduino/NRF24L01, would certainly avoid some possible frustration and might give you enough understanding of each project to enable you to get them together as one project later.