If you want to integrate the two systems so that you have your existing users continuing with the PBX and new users working with IP phones, then use E1 QSIG between the IPT gateway and the PBX giving you a trunk of 30 channels. Then you can slowly phase out the PBX users over time to protect your investment made on the phones.
If you want to scrap your PBX and just use your analog phones, I'm guessing multiple VG248's (5) as it would make sense to have 100 ATA186's.