Yellow stuff = crap.
MK7 Performance Pack/Cupra R/MK7 R = great.
MK5 R32 = effective, but effin' heavy.
Whats the going rate of a Mk7 R upgrade kit? in comparison to circa. £500 for MK5 R32?
Not sure, probably around the same.
I bought my stuff new. I don't like second hand brakes or engines as you don't know what abuse they received from previous owners.
Brand new Cupra R 280 (same as 7R/S3/PP) calipers & carriers with pads from ebay for £350, then just added genuine Clubsport S 2 piece discs to that, which were another £300 iirc. Not a cheap option, but a very effective one, and all OEM for reliability, fit and finish and life span.