Worth considering your options for front brakes rather than just grabbing a set of brembos from JPP. They used to sell for about £600 as used calipers/discs/pads. I had the same decision when replacing my 4pots, and if the brembo discs/pads are half worn, it might actually be cheaper in long run to get a ksport kit - which will be more powerful too
My concern with buying second hand calipers is that they might need a refurb - how long have they sat off the car, and are any of the pistons sticky?