Coming from a Vauxhall background, i've owned and driven many Xe's
To be honest I wouldn't worry too much about finding one with a coscast head, it's a bit of a myth about the performance gains they have over a GM head, you may find that the finish on the porting is slightly better on a coscast head, but if your going for power, you'd most likely have this done anyway.
As for the porus issue, it is true that a coscast head is less likely to crack and go porus, BUT it is normally the case that if a GM head is going to break, it will do within the first 20 - 30k miles so unless you somehow find a crazy low mileage car, I wouldn't worry too much.
Most of the problem with heads on older engines is due to people over-tightening the head bolts, where the bolt runs very close to the oil gallery, and then cracks it.
The engine you really want if your running std injection. is from an early Astra mk2 GTE, which should have the coscast head and also runs a lighter flywheel and different engine map due to the lack of CAT, these oftern see over 150bhp on std trim.
Or if you fancy being different, you could go for the cavalier / calibra turbo lump, virtually identical to the XE, and has 205bhp std and very little turbo lag.