2010 Mini Cooper - repair or replace?

My beloved 2010 Cooper Classic needs new rear brakes and significant AC
repairs, that will cost more than 50% of its market value. It only has 75,000
kms on it and is otherwise running fairly well. It goes through oil rather fast
and the battery dies if it sits too long. I'd like to keep it, but I'm worried that
serious, very expensive things are going to start going wrong, given its age. I
asked my mechanic whether it was worth doing the repairs and he said it
was "50/50". Anyone have experience with low kms/high years, and what I
can expect?