8 core is definitely the way to go the price difference is not that much either compared to 6 core,Make sure you buy quality CQR alarm cable pure copper preferably, there is a lot of soft brittle rubbish alarm cable out there.
As for upstairs it depends how easy or likely someone can get access to upstairs, every property is different, Only you can be the judge of that, As a rule of thumb you generally fit a pir to cover the upstairs landing.
As for upstairs it depends how easy or likely someone can get access to upstairs, every property is different, Only you can be the judge of that, As a rule of thumb you generally fit a pir to cover the upstairs landing.