Of course, I won't get exactly similar needs 
I mean here is currently 23:34 and still didn't find something similar enough to each question I mentioned, since maybe 18:30 or something... and my schematic has been done 2 days ago and if I came here it is because google on this was not “my friend” during these 2 days (few hours each only of course)
BUT no ; it is really not a leak of search and NO I am not just too lazy to search 
I know I need to adapt things but I need advices before continuing I won't work in bad directions… 
if I can avoid it
Currently, I have no choices on
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Powering is done by battery because I can not have access to any kind of plug nor to the roof of the mailboxes zone
-
communication cannot (for version zero) be made buy another way (using parts I have in stock) so 433Mhz module
-
Detection .... here again is related to stock so ... ultrasonic sensor
so NOW Questions remains the same as the main post 
Before I go forward on that idea I want to know
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the concept is OK for that -
the arduino model to use (I have unov3 & nano available currently) -
is current consumtion a problem with this concept (boards limitations ?) -
is this dispance OK for 433Mhz modules (after replacing the antenna maybe)
Until those 4 points are not answered (even partially) I won't go further 
see that tomorrow