The ST accelerates so quickly that my husband's Zetec S, which I also drive, seems steam driven in comparison! If you get a chance to drive an ST, go for it!
I think the facelift is a great improvement. As for the S, we have one - handling is great but performance is nowhere near the Puma. Ben also found that out after he parted with his!
The main cost of replacing the engine is the labour, plus other parts such as cam belt etc. It cos me over £1,000 iirc on top of cost of engine and carriage. Not cheap.