Suggest you look at web sites for PBX suppliers eg. Mitel, Siemens, Nortel,Ericsson,Lucent. These should give you information on their PBXs to cater for 250 and 85 extensions.
From traffic figures (erlangs) you give, I reckon you would need 35 circuits between Chicago & Naperville, with 82 PSTN circuits from Chicago and 70 for Naperville. These could be ISDN ccts. ( I think these are called T1s in the US and each have 24 useable channels (circuits). These would need special cards in the PBX.
Hope this helps.