"There will call upon you to-night, at a quarter to eight o'clock," it said, "a gentleman who desires to consult you upon a matter of the very deepest moment.
Hi Pructus,

These are both correct, though a bit old fashioned in structure.

In the first one, there's quite an interruption between the subject and verb. Think of it as "A gentleman will call upon you tonight." There will call upon you tonight a gentleman. There will be a gentleman who will call on you tonight.

In the second one, your translation is correct. If you've ever studied Newtonian physics, think of "moment" in the physics sense. It's similar to "momentum." A significant or substantial or important moment in physics would be a characteristic of an object which has substantial mass (weight) and inertia (movement). That is, it's heavy and it's on the move. It has the potential to do something.

But, as I said, your replacement of "moment" with "importance" is correct.

Best wishes, - A.
I see, Avangi !!

Thanks a lot.....