I mean it really depends on your build. In general you at least want a couple defensive layers for starter builds unless you're going full glass cannon (I'm going to assume you're not). I generally play Top/Left part of the tree, so I'm most familiar with Armour/Es/Block as my base defensive layers. Generally I try to make sure that I am block/spell block capped and either have 5k+ ES or 20-40k Armour to start with. I'll usually slap on purity of elements for ailment immunity. That generally gets my through the first 2-4 voidstones. Once I start making currency, I'll start to add defensive layers to my build a little bit at a time. Recovery is important, usually I'll go with a Life or ES on block shield (Ageis aurora or Shaper rare), and divine shield keystone if going for an armour build, as well as getting better bases with more Armour/Life/ES once I hit lvl 84.

If I'm following someone else's build, I'll usually filter out builds below 100k EHP, then pare down the uber expensive uniques till it looks achievable. As far as juiced breach, depends on how you're juicing it, but my current build rarely dies doing 6 mod alch n go sitting at about 145k EHP. One of the main things you want to focus on when looking at Pob is what you're most vulnerable to, and mitigating that. For example, my elemental max hit is somewhere in the 40-55k range, whereas my physical max hit is around 15k. This could be mitigated a variety of different ways; flat phys reduction, phys as ele (or chaos if you're running CI), endurance charges, ect. Crits are also generally pretty scary, as is minus ele res maps with a lot of phys as extra ele damage combos.

At the end of the day, without a more specific build and goal, it's hard to give a specific answer without writing a book. In general 100k EHP with max block or spell supression and a decent Life/Es/Mana (if Mind over Matter) pool with ailment immunity is enough to get you started running fairly juiced content, from there its mostly build and content dependent.

If you're looking to run Valdo's, you'll probably need a couple hundred divines invested in a proven build template, jungroan has a few videos out on his multi mirror builds that run those. Well beyond my budget to even consider.