mate tbh i would leave it in the hands of a mechanic, deffo dont want your suspension goin wrong, bad news!!

if it helps i got lowering springs put in my ibiza for around 70 quid so i wud say your maybe looking about 130 ish for the coiovers to be fitted.
 
muddyboots did a guide for replacing shocks on the mk4... you have to do as the guide states to put coilovers in... search muddyboots in members and im sure theres a link in his signature... "mk4ibiza" or something like that!