As Dave says, there's nothing particularly different between the ST170 and a standard 2.0L zetec as far as turboing is concerned, except for the higher standard compression. Easiest way to sort that and get a nice strong bottom end, is buy an FRS short block from Ford - £800 trade. Then you just need a turbo, manifold, frs / area six / adrenaline / whoever inlet manifold, sensors, ECU, FRS fuel pump / rail / regulator / injectors, standard focus return line (st170 is returnless high pressure fuel system), intercooler, boost/water/oil plumbing. You'd probably also want a boost controller and some gauges, bigger brakes, posher suspension, better tyres... So it's not cheap. You CAN do it for 3.5-5k, if you're lucky/patient finding parts second hand and do all the fitting yourself. Otherwise, budget around £10k.

Chris