First, let's start on the legal side of the equation. If you are in the US, you are required to have a license for your business radios as well as for your planned repeater. You do not get to pick the frequencies, they are assigned to you by an FCC approved frequency coordinator.
The license application is not particularly difficult or expensive but it is loaded with terminology that can be a barrier for the uninitiated. You would probably be better off to use a professional service the first time through the process to avoid having your application rejected by the FCC. You generally have one license that covers all of your radios, the repeater, and at least two assigned frequencies.
On the technical side, the transmit and receive frequencies will be 5 MHz apart. This is so that the repeater can simultaneously receive and transmit with minimal interference to itself. Even then, a very high isolation filter, called a duplexer, is required so that the receiver in the repeater is not "desensed" by its own transmitter. Desensing refers to the repeater receiver losing sensitivity for the desired signal due to a strong interfering signal - from the transmitter in this case. The coax cable used throughout must be at least double shielded to further prevent desense.
The repeater must sense a desired signal on its receiver and turn on the transmitter. It must then turn off the transmitter a short time after the receiver loses its signal. The repeater must transmit its FCC assigned license number, generally via morse code, generally during every 15 minutes of operation. All of these functions are typically performed by a piece of hardware called a repeater controller.
The transmitter used in a repeater must be FCC certified for the service. Generally the transmitter type is chosen to survive the tougher duty cycle of the repeater and to minimize any interference to the receiver due to phase noise and IMD.
I hope this introduction makes you aware that a successful repeater installation can be rather expensive and it requires a fair bit of technical expertise in addition to some expensive test equipment to get it to function properly and legally. In addition, the Chicago area is very "RF dense" which can raise many technical challenges. You may wish to get someone with the appropriate experience involved in your project.
As an alternative, see my suggestion for WiFi radios in my answer to your other question.