All air filters are easy to whip in and out, half hour job tops. Which one you get comes down to price your willing to pay.
172 Springs? Dunno about them fitting at all but I doubt it and even if they do I dont think it would benefit you in anyway really. Im guessing that as its an import you were unable to get the Cup suspension, therefore if I were you Id look into getting some Eibachs, a lot of the Cup drivers on here have them and they are certainly the most popular.
You could of course get coilovers but again it comes down to Budget.
£12,500 is about the going rate for an import, perhaps a little high. Remember you wont get the chassis number etched onto the glass, you wont get a Cat One alarm (£300) and you MIGHT not get the mats. You wont get locking wheel nuts either so Id invest in one of those.
All in all fiarplay youve avoided the wait but I got offered one for £13,500 from a main dealer and others on here got main Renault dealers down to £13,200, one I know of to £13,050.
Oh and one more thing...
http://erc.qmuc.ac.uk/cliosport/gallery/full/1092088914__capslock.jpg