Would need to know what sort of budget and what sort of power you are after to make a recomendation really.

Golden rule is that bigger engines are cheaper to get power from of course, all be it they then cost in terms of weight.

XE's are getting long in the tooth now, hard to find, and there is a lot of demand for them, so prices are only going to go up from here on in I reckon.

Standard S2000 engine is an awesome way to get more power than you would if you put 5K into an XE!

I think the S2000 would be top of my list for something like a Mk2 escort etc, well unless you start looking at V8's like the LS1 etc