Hi mac 7,
Whichever a 235/40 or 255/35 as long as is 18". Thing is hard to find a reputable wheel in 8.5" or 9" wide wheel in 18". Most common size is 8"X18." OZ Racing has an ultra light wheel Alleggerita at just over 7kg for a 8X17." Regular 8X17" is about 12kg.
IMHO, go for a 8.5" or 9" wide wheels with 235/40 or 255/35X18. Order car with ACC & throw in a set of lowered spring from H&R or KW. They should have a set of these springs soon for the "R." H&R has a set of lowered spring for the TTRS & is compatible with its "magnetic ride," Audi's version of ACC. Lowered by about 25mm. I would use 19" on a TTRS as it suits the car better. 18" is standard fitment on the TTRS just like the "R" but uses a slightly wider tyres.