The refurbished rooms at the IC use the same style of furniture, not the actual same old table and chairs. New rooms are lovely, I've posted pics on this forum FYI.
Personally - while I like the Indigo brand in general - I can't see how the rooms at Indigo Paris are nicer than the ones at the IC. More modern, sure, but pretty pokey...and the plastic flooring is not my thing.
As an IHG Plat I'd say the OP has a good chance of being upgraded at the IC (probably at the Indigo too, to be fair) so wouldn't be in the smallest Classic rooms.
As for best property - in my opinion, HI Notre Dame is in the best location as a tourist, but none of the hotels mentioned are in bad locations.
Aside from price and style preferences, I think transport should be a guiding factor in chosing. If the OP is getting to central Paris by public transport, Notre Dame first, and IC/Indigo second are winners as they're close to RER - handy for those arriving from airports.