A williams gearbox would be able to take 250BHP with out any major issues. BBPTs clio 16v turbo demo car runs a std williams box. Any more and you may have to concider maybe custom quaife one?! But then your talking big £££. BBPT turbo 16v is limited due to the gearbox, tho some say the chassis cant really deal with any more power what with it being FWD and all. If it wasnt for this, itd be running something in the range of 300 - 350 BHP.
As for the clutch, speak to BBPT - there are many to choose from.