Hi Jon
First of all if you are cutting down that number of trees you are going to need a felling licence from the FC. They will want to know what you are going to replant with.
Regarding the roots, I think your options are:
Fell as low to the ground as possible and live with the stumps.
Use a jcb digger to dig them out, don't expect to get them out with a small mini digger. A root ripping tine on the digger is helpful.
Hire a stump grinder, I have never used one but I would think it would be a slow job.
Get a large bulldozer to push them out .
I have dug out spruce stumps with a 4 ton rubber tracked digger, no ripping tine, I would not want to do 100 though.
John