I fitted a Dunnell Zetec to my Escort (now Sambo's) after a lot of consideration between Zetec, N/A Cos, BDA, XE and even Lotus Twincam. In the end, I chose the Zetec because it was the best package overall for what I wanted. It is very light, and had the best low down torque and very flat torque curve. Being a mainstream Ford motor, it was also very easy to get a hold of standard parts cheaply. It was however very far from the cheapest option, though I did mine a few years ago and maybe prices are ok now, I dont know.
I think it might be easier to get higher bhp figures from an XE from your £1500 than a Zetec, but I have never owned an XE, so others will be able to advise you better here.
Hope this helps!