when to buy Apple Care.....in Portugal !?

Hi guys

i'm about to ''bite the bullet'' & get myself a nice 2013 27'' i7 iMac, 3.4GHz, then get the 32GB RAM, etc, etc....& i still am not sure if i want to buy it new or second hand - a friend is selling is, he bought it last May & the price is VERY tempting, 600€ less & his already has the 32GB !!..

this time however i think i want to get some extra peace of mind & buy an Apple Care ''plan'' for it

my doubt is : can i buy it like on the last month or week of it's warranty or do i have to buy it simultaneously with the iMac ? & if i buy the second hand one can i still get an Apple Care for it ?

to further confuse matters & hoping some portuguese forum users might be able to help on this, since in Portugal warranties are 2 years by law, can anyone shed some light if the answers to the above questions still apply or does Apple ''impose'' like a time limit for purchasing the Apple Care ?

many thanks & regards to all

Nuno