Never brewed an all grain stout/porter.
Any recipe suggestions for my first attempt ?
As you have found, it's complicated, you first have to define what a stout or porter is. Yes, originally stout porter and porter were just different strength partigyles from the same mashes, so used identical grists, but they have evolved since then. Just within the British Isles, Dublin stouts are very different to London stouts (which are sweeter and don't use roast barley as a rule), and other Irish stouts are somewhere in between. And the pervasive influence of 4.2% Guinness means that British commercial stouts on average are now weaker than porters, which makes no sense historically.
And then you have all the variations that have happened outside the British Isles.
But here at least, the benchmarks are Guinness and Fuller's London Porter. Although everyone thinks of roast barley as the defining ingredient of Irish stouts, Guinness didn't start using it until the 1930s, although others had started using it before WWI. Typical Guinness clone recipes are 70% pale, 20% flaked barley and 10% roast barley, but you can play around with other unmalted and roast ingredients, US recipes tend to use a bit less flaked.
We have a pretty good recipe for
Fuller's Porter (and the new imperial version with glucose + treacle) - 75% pale malt, 14% UK crystal 60L, 10% brown malt, 1.5% UK chocolate to 1.056, with 37 IBU from Fuggles or similar. Use a characterful yeast that doesn't attenuate too much - Imperial Pub is ideal, or something like WLP041.
Brown malt is the defining ingredient of London porters but some people appear to not get on with it - I'm not sure if that's personal taste or reflecting differences between maltsters. Personally the Fuller's porter is one of my absolute favourite beers, it's one I will go out of my way for if it's on cask.
Historically the fancy ingredients like Otter and EKG would be saved for the premium pale beers so dark beers (in the UK at least) would be made with second-line ingredients - ordinary pale malt or worse, lesser hops like Colgate and Grape etc. So don't sweat the ingredients too much - Nugget will be fine for bittering.
What PH range am I seeking ?
You end up in the "normal" range for mashing, the point is that dark malts are more acidic than pale malts so if you have soft or RO water then you can end up with a mash pH below the optimum for mash enzymes.