Terry, given how easily you've managed to acquire Astras with expensive problems, then buying one with a suspected head gasket issue is daft. It would only be worth considering if you were going to be given the car and that you knew the rest of its history. Even then, due to the time and cost in fixing the problem you know about, I would suggest that you pass on it.
Your method for buying cheap snotters is all wrong: don't search for a specific model but only look at cheap cars that are local to you, meet your realistic requirements and have a long MOT. Be very critical of durability, and buy what comes up that is on the road and likely to stay that way for the length of the MOT without anything done to it. This will give you the opportunity to decide if it's worth spending money to improve it. For instance the last two times I walked to bell practices, I passed a tidy '52 Fiesta for £550 - petrol, manual, five doors, nice spec(it was a Ghia), good condition, smart metallic blue, 4 good tyres, 6months MOT. It was parked on the main road, and was gone in two days. That's the sort of thing you should be looking for.