It doesn't really matter, imo anyway. I'm to the point where I only buy Subarus that have blown head gaskets. If they are in running condition, they hold their value remarkably well, but when they are broke, they are cheaper than dirt. Take my Legacy wagon for example, I bought it a few weeks back for $500, blown headgaskets. Now, I haven't had a chance to fix it yet, but Sean (crossle) and I have done enough of these things to know that 8 hours and a few hundo worth of parts later I will have a daily that runs like new and will be worth several times what I paid for it.
As for headgaskets, I am pretty sure there is a schedule, but I don't remember what it is.