As a baseline that has a lot of flexibility based upon ally protection, other mitigation:

somewhere in that ~40k HP/~3.5k DEF range for killable teams. 

So if I can keep up Ally protection all the time it can be less? Like 35k HP/2.5k DEF?

Ally protection effectively halves the damage you take. The "sweet spot" for DEF is typically either ~3200 or ~4200, depending on just how much mitigation you need - 3200 gives you about 75% damage reduction, 4200 gives you about 80% damage reduction. Things fall off quite sharply after that.

Most people aim for the 3200 mark. If you assume the person placing the AP is "safe" (generally due to just higher defensive stats), everyone else can essentially aim for half the damage reduction - which is as little as 1500 DEF. And all of these numbers are without DEF up. In other words - once you have AP active, you can more or less ignore DEF completely.

What remains is the stun. If you're able to make sure your squishies are never targeted, you can more or less ignore DEF and HP altogether. If they need to be able to survive stun, though, then you'll want to make sure that they have at least 45K hp.

Thank you, that helped a lot!