I4 is deffo a bit harder to fit to a mk4 than a Zetec, i've done both. Plus if you kill a Zetec its far far easier and cheaper to source a replacement engine.

I would suggest a 2.0 silvertop Zetec with 130 spec management and a decent exhaust and 4 branch - I had this before my ST170 lump and it was a brilliant conversion - totally reliable, very quick, great on fuel and sounded ace too. Could prob do it for under £500 if you tried too!