They do a kit specifically for fitting an uppipe mate. An Sti uppipe used is like £60, Brand new decat uppipes are like £150-£200 ish. The labour because of where it is will not be cheap that is if your mechanic doesn’t throw a hissy fit cause you want to supply your own parts I’ve had this loads in the past til I found the guy I use now. I’d send your scan tool back and say money please buy one that will work and clear the code then enjoy your car for a while while you save for a few bits and a map. You can probably achieve this with like £500-£600 tops if you go to the right person. Again mate you really need to know what that code is so you don’t pay for a mechanic to guess and just throw a grand down the toilet for the code to come back on the way home. By the sounds of it they have read the code and plan to just throw a new cat at it and hope for the best. I hope this isn’t tainting your enjoyment of your car mate There are so many who have gone through this with Scoobys me included.