The standard leads are very good, better than the aftermarket so called upgraded versions so i would be suprised if these are at fault, buy ford plugs they are the best if not a bit pricey at about £9 a plug, ford parts uk do a good deal on 4 ford OEM plugs for about £25 plus VAT
I doubt its the above, it will be the coilpack or fuel pump, im always suprised to hear of all these ST170 engine problems, apart from the clutch mine has been golden for 3.5years now not a single issue