I am looking at buying a subaru. Forester/ Outback, I am still
undecided. I have read  a lot of reviews with complaints about head gasket
and seal issues. Is this a rare occurence? Is there any (safe) years?

as a VERY general rule, avoid late 90s to 2000 or so 2.5l DOHC engines. (severe HG failures if not rebuilt with MLS gaskets)

 

I'd suggest used turbo engines are also very risky, the worst being around the 2007-8 years, depending on model. (oil screen problems)

 

used cars of all type have issues. Also, even the above problems can be mitigated if caught soon or 'properly' rebuilt.

 

plenty of other little gotchas here and there....what are you considering?