These answers will all be my personal preference
So my Questions:
1. Would you take bigger Display or higher PPI for the same price?
I would take bigger display but my love is with 27", it's just perfect, and definitely would get more PPI for the same price
2. Is 2560x1440 on 24" useable without scaling?
It's quite small to be honest, though my experience is only with 27" 1440p for me it's perfect and I can't think of anything smaller at that pixel
3. As a base for a future 2 or 3 Monitor Setup, is 24" to small as a centre?
24" is perfect for surround, it's already that immersive
but if you plan to multi monitor (not surround) it depends on personal preference. I tried 27" 1440p centre and one 24" 1080p (both vertical and horizontal) once, didn't love it. I'm now with 27" 4k centre and 27" 1440p on my left, perfect for me (different scaling)
4. Is 4K worth it on 28" when considering my GFX Card might need to be upgraded or should i go with 2560 on 27"?
This is debatable , I love my PB278Q, honestly more than my LG 27" 4k. But I love to game sometimes and this LG has 4k, IPS, sRGB, and Freesync! how can you beat that for gaming?
5. My Desk is mid sized and i sit around 1m away from the monitor. Which screen size would you recommend from a ergonomic standpoint? Is 32" to much?
at 1m , 32" is still OK
for reference I sit around 60-70cm at my 27" and it's perfect for me
My piece of advice
1. Always go for IPS
2. Suit your budget, suit your use. If you like gaming get high hz or freesync/g-sync , it makes the difference
3. Get something with good warranty in your country
4. Try not to go the Korean cheap monitor like QNIX, I had the once and you pay what you get for in terms of build
from what I see, that Dell 30 really suits you

Can't go wrong with Dell
how about dell u2715h? doesn't suit the budget?