Just read what I wrote again, you cant borrow someone's rstuner as it will be locked to their car, as said you can unlock it for 49 euros but it will then lock its self to you car as soon as you plug the unit in. The best thing is to keep an eye out in the for sale section and get one secondhand and get it unlocked for you car. If you look at fastchips website there are various maps, all you do is see what ecu code you have and stick the 98 ron of group N map on and away you go. You can pay fastchips for a custom map depending on what mods you have done.
The other option is to get the car set up on a RR which to be fair is probably the most accurate way of doing it.