While it's great to ask Trulia and Realtors, your best bet is to get at least 3 licensed, reputable HVAC contractor estimates. They will be able to explain the entire process and expected cost, work, etc. with you. You will learn so much from them just from their initial inspections. They should be FREE estimates. Ask before they come out if the estimate is free. Don't let anyone charge you or push you into signing a contract.
This is a relatively popular upgrade for our area due to older homes and don't forget you will increase your property value along with possibly reducing you energy bills. Good luck to you!
Cecilia Sherrard & Team
RE/MAX Northeast, Ohio
Tari Torch Sweeney