I am planning to replace my York furnace with a Coleman Echelon 9.C Modulating Gas Furnace (model FC9C-UP).

I would like to know if Coleman is a reliable brand and if I should choose the same size unit for the Coleman (100,000/95,000 BTU input/output) or can I go with a smaller unit (80,000/76,000 BTU input/output) now that I increased my attic insulation to R-30?
Thanks

Having been a Coleman installer many years ago and being involved in this industry for many years. I would not be worried about this equipment. The equipment is quite good and you should not have any real issues.

However with that said. You must look into a few other factors before you move ahead.

How many Coleman contractors are in your area? Or are you sort of stuck with this company if anything goes wrong down the line? Is there a local distributor in your area for parts? While most larger contracting firms can work on the furnace, they need parts quickly if something breaks.
What is the rep with the installing contractor? Have they had issues in the past with other homeowners?
I have said many times before on this site. The contractor can use the best supplies in the world, but if they do not understand how to do this properly then the material will not end up doing its job and the result the manufacture of the furnace and AC system will end up being blamed.
Do your homework on the installer, The equipment is fine stuff.

Forgot to add, As far as sizing the unit. The output of the system determines what unit you put into your home. The contractor needs to size the home properly before choosing the furnace size. Typcially because of the higher efficency of the newer units the furnace can run longer periods of time for greater comfort and lower utility costs. But its a balancing act to provide energy savings and longer run times to get this efficency and comfort levels.
So to suggest what size unit you need on this site is not a good idea. Get the house sized for the equipment then choose the size. If you do not trust the contractor contact your local utility company, many do this service for free or for a low fee.