I went through ISSA years ago, but this article talked specifically about the pros and cons of those four certs.
Really doesn’t matter. No client is going to ask where you’re certified through and gym management just wants to know that you have some kind of certification. I actually lucked out that my first gig was at a gym run by another ISSA-certified trainer who contacted me after she received a list of local people who recently completed their certification.
Definitely take a read through the Personal Trainer’s thread and bump it with any other specific questions.
It’s good that you recognize this ahead of time. A lot of guys fall into the trap of expecting to train their ideal clients or versions of themselves right out the gate, which just doesn’t happen.
So knowing your most likely demographic, I’d brush up on your toolbox of mobility drills, practical real-world nutrition tips (depending on if you’re allowed to tackle nutrition as a trainer), and get familiar with working around common pre-existing conditions like arthritis and hypertension.