You will be hard-pressed to find factory ammunition that is not safe for a modern factory spec 9x19 gun, unless it is some Reloadin' Ronnie's +P+P+ insanity or Danny Dremel's "improved" pistol.
Steel cases will not expand/contract exactly like a brass case, so you may get the occasional malfunction, which is free training for real malfunctions. Buy a couple hundred, shoot them up, and if your failure rate is acceptable go ahead and stock up.
I shoot steel or aluminum cased ammo at my outdoor range, where finding all my brass is about impossible and 50% is doing pretty good. Beyond some steel-case not agreeing with my PPS during initial break-in none of it has been a major problem and most of it was cheap at the right time to stock up. I buy cheap 9mm steel-case or Blazer Al when I spot it for this very reason.
I don't understand the hesitation, commercial ammo manufacturers know that some asshat is going to shove their cheapest range ammo into grandpa's 80 year old Luger and sue the ammo maker if it goes "boom" instead of "bang" ... thus target ammo is loaded reasonably lightly.
All bets are off when you see terms like "+P" or "+P+" on the box, and steer clear of the crap in baggies from the local "professional" reloaders, and if you have an antique (I notice you didn't list the age / round count for your P99 ... but I suspect it isn't a priceless antique) stick to the light loads, avoiding "+P" and the like.