Two plumbing companies I would recommend include: Aleck Plumbing 708-799-4670 and Morning, Noon & Night Plumbing: 773-880-0072. Heat & Air I would recommend Assured Air 708-839-5760.
Angie's List is a great resource for finding contractors for rehab and construction. Also try the Building Owners and Managers Association (BOMA) here in Chicago.
Stop by your Alderman's office to talk with him or her about rehabbing in your neighborhood. Ravenswood straddles four different wards so you'll need to carefully search for your alderman. Do a google search for Chicago Alderman or Chicago Ward Map. You might be in Ward 32, 40, 44 or 47.
A qualified Realtor will have references for attorneys to help you create condominium associations, find accountants and - of course - locate great properties.
Next, find a Realtor who has experience working with people who rehab properties. That Realtor will be your best asset.
You are very wise to ask the questions you asked and I encourage you to continue to research for your answers.
Once you have an understanding of what it takes to rehab, with the pricing in your mind, add on the taxes and permit requirements specific to Cook County, (and there are several).