hi what is the esp? in short the st170 is the basic zetec/zeta lump with uprated high comp inturnals, agressive cams and vvt, with high flow down pipe and cat, and larger throtle boddy and air intake,
(think the esp just means it came with the anti skid package)
as cheep upgrades:::
the brakes and suspension is worth nicking of the st170(new st shocks tho)
if your car is a zetec silver top you may be able to nick
the air snockle, air box and maybe even the throtle body off the st170?,
the exhaust down pipe 4/2/1(is a great upgrade) andthe st cat or add sports cat, these wont add much in way of power on ther own but may liven it up,

other ideas.
dont know if the st cams would fit but - kent cams or cat cam or piper cams could do a kit, (dont know if this would need remap but would be best)
adjustable vineer cam pully, (works best with remap)
uprated injectors rs or st -(would need remap)
the remap,
this would put it very close to the st170
light weight flywheel and sprung clutch for more omph check ttv http://www.ttvracing.com/FordSTrangeflywheel.html

also check out http://www.pumaspeed.co.uk/Focus