Hey Lucas,
Yeah I'd go at least 75, my (door) desk is 75cm deep (and honestly if you have the room lengthwise it really is a better and cheaper option) but I'm using a 21.5" iMac and due to the base the furthest back it can go means that the screen sits around 20cm from the back of the desk and the wall. this means you have 55cm of arm room but.. honestly, I lean back a fair bit too as even with the smaller iMac size, it still strains my eyes a little being so close.
So if you are *set* on the 27" I would look at at LEAST 75cm in depth, I know that in work I use a 27" and it's on a corner desk but it's a design studio so our desks are around 90cm deep to allow room for working on a3 sketchpads etc too!
It's entirely down to what you are comfortable with, but if you take into account that with a 60cm depth you're looking at 40cm (or less, not sure if the 27" iMac has a deeper stand depth) between edge of desk and screen!
By the way, where are you based, if you're near London I have a white ikea Vika Amon desk (the one you initially looked at) that you can have for free if you can pick it up, that way you can take it and see how it works out for you, if it fits, great, if it's not deep enough then you've not wasted any money!
PM me if you're nearby and want it, couldn't find any info about your location from your profile on here!