Either PADI or SSI, you'll need EFR/CPR and Science of Diving before doing Divemaster, you may be able to do the CPR with the dive master course. I'd look at where you want to be or work and do the dive master training there and don't do just the class, intern or work for a bit too - you need the local experience. Some places do the instructor training with an internship too - it gets you teaching and experience locally guiding.
I'm 53, was SSI up to doing Dive Master with PADI, starting instructor internship in a few months.
I'm 53, was SSI up to doing Dive Master with PADI, starting instructor internship in a few months.