Oi, Dan, don't be talking sense in here!
I've not noticed any problems with the 1.7 Puma, never met anyone who's blown the engine in one, and don't know any welders who're snowed under by 'em.
The 1.7 lump was detuned to 125 bhp or so because they were going to put the "full power" one in the Racing Puma. Then they decided that the Racing Puma couldn't have full power either because then it'd compete with the RS Focus so that ended up stuck with 150 bhp.
They're like engines built by a certain other company that shall remain nameless - if you don't check the oil regularly and service them properly, they go bang. With the "other company" products that isn't the fault of the car, it's the fault of the owner, but because the Puma is a Ford it doesn't appear to work that way.
Having said that, I've yet to visit a scrapyard that has more Pumas than "Other company" produce. :-)