Use FXS gateways. You can take a look at Sangoma, Grandstream and others.
There is a lot of differents equipaments.
Usually with something like this we would run channel banks of of T1 ports.
The Sangoma Vega 3000 and Vega 5000 would do this very well and what they are designed for.
one thing I missed in the original post was the single PCI slot. I am with tony get 2 of the VEGA 5000 and that would service all 100 lines without the need for add in cards. As a bonus those give you a couple FXO ports too in case you need to bring any pots lines in to your system.