17's probably won't fit, also the centre bore is bigger so you'd have to make a ring to keep the disc centered.
tried finding alternatives at alot of car makes but none fit the bill really
wouldn't say much cheaper though as you would need longer caliper bolts, spacers, redrilling the discs, filing the calipers for the bigger diameter disc and fab a ring for both discs.
there isn't any aftermarket set available for the 250 yet so discs are about £250 new, ad to that the cost and time of the rest and you'll be heading towards the reyland 330 kit territory price wise.
when the day comes that 250 discs will be more affordable i'll probably be doing this but for now the original ones will do.