I am looking at 3d printing a round object that is 700mm in diameter and 200mm in height?
MPCNC or MP3DP?
Yeah, that’s what I was looking for. Great thread that one with a lot of quality information. 
I have all the parts except the core for the MPCNC.
Conduit vs extrusions?
I would have a z workspace of 250mm.
Is this bad for rigidity? Perhaps an extruder doesn’t need rigidity?
Any.machine needs rigidity, but it’s a matter of how much.
With the massbof an MPCNC, you will have to print more slowly than gou might otherwise like, but it ought to be doable.
I would be tempted to try to build the printer… if I could find 850mm MGN12H rails… longest Ive seen is 600.
What speeds do you think would be reasonble? And what resolution could I expect? Is it 0.1mm or much higher?
I found longer rails from 100 to 1000mm.
Do you think that the quality would be better with the Primo or the 3d printer?
The 3D printer is more likely to have good 3D printing results, in my opinion. A lower moving mass for the print head, and it being more purpose designed for printing is far more likely to yield better results.
The Primo is capable of being a large format printer, it’s been proven.
An 850mm MGN12H rail might have some sag, which would be my only worry, but it should be consistent, and mesh levelling will take care of it.
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I have looked around and 510x510mm seems to be the biggest size of bed out there in normal quanties.
Do you see any problems with a larger bed then 510x510mm? I would like 750x750mm.
EDIT: I guess I could go with a mirror or a glass bed.
More than! I’ve got a build area of 1250mm x 980mm to allow tiled use of full size sheet materials (typically 1220 x 2440).
You’ll need either solid bars or thick walled tubes but other than that it works fine.
Due to leverage you cant move the core around by hand quite so freely or shift the whole gantry by moving one bar as it goes out of square too easily, but when running on motors and tuned properly its been flawless for me so far.
That said it is absolutely massive!