How should I start my training so it's not too hard on my body?
No matter what you do, it will be hard on your body, but there are things you can do to make yourself more resilient. First off, I would start a phased workout plan that involves different focuses over time. So I would start with running then go to swimming, then Cals, then rucking, and so on. Increase the weight you are able to carry and make your body used to it in order to reduce injury chances later on.
I was wondering what physical benchmarks I should be at before starting training, as well as what gear I should be buying?
In order to start the past program there are not specific benchmarks but I usually tell people as long as they can pass the AF PT test then they will be just fine. Also, you can repeat the workout as many times as you want for up to a year from your purchase date. Equipment recommendations can be found on my website here or on my amazon page here.