Thinking about "upgrading" to K30/K50/K70/K3II, suits me?

Hi.

I currently shoot MFT (Panasonic) with a bunch of Pentax-M PK legacy glass. And a few M42 to top it off. My DMC-G3 is getting on in years and I'm thinking about upgrading. I would like my next camera to have IBIS, Focus Peaking and preferable weather sealed. Naturally with that set my research lead me to Pentax, as you all probably guessed.

But, I would primarily like to have a mirrorless system for the size and the EVF (as you can't use focus peaking on optical viewfinder).

So my list of questions to you is:

1) Is the Focus Peaking easy and intuitive to use on the LV on these cameras? Is the display good enough for it and for general use in sunlight?

2) I'll mostly shoot people (kids mostly) and some "nature". Possibly some "planned"/"studio-like" photography to top it. Is there a real difference between K30/K50/K70 that I should go for one or the other if there is a real price difference.

3) Is the difference between the Kxx and the K-3II worth the extra money? (I could get a K50 18-125mm kit for 900€ and the K3II 18-135mm kit for 1300€.)  The 1300 would really be stretching my budget though and ideally I'd like to keep it lower, but I *can* probably go there if the difference is great enough.

4) Ideally I'd like a single "walkabout" AF lens for everyday photos and then just use the legacy glass I have for more planned purposes. Is the 18-135mm WR (close to ideal focal range for my needs)  a good enough lens for that? How's the AF speed and general build quality?

5) Going forward, I'd like to get a TTL flash (I have a few manual ones and they'll do, but...) Is there a good flash out there for general use for Pentax? Only thing I could find was a Metz 44 AF-1 for around 150€ from ebay, but it seems to not be available from local stores at all?