I would check with a local HVAC (Hydronic) wholesaler or installer to see if you can still obtain them {be prepared for them to cost almost the same {probably more when you factor in labour} as replacing the entire unit.As for using PEX, I'm not certain if the heat-rated PEX (used for in-floor hydronic heating which runs around 140F) can be used for a boiler and radiators (which typically run around 180F).  

I would find out what the utility costs are and if the rate you see on rentometer include utilities.

My only concern is being new to the game and not having the experience that they typically look for.

Please share your experienceWhat would typical water/garbage/gas bills look like for 3-4 unit multi family building in Chicago / Cook County / Illinois?

Surround yourself with like-minded individuals and utilize this network to grow your contacts and education.

I ended up getting 3 roommates and collecting $900/month in rent while only paying $575/month mortgage plus utilities.  

Also, are there any back taxes on the property you would have to pay for, any backed up bills that are due to the city/utility companies?

Typically this is because a borrower gave the property back to the bank with a deed lieu of foreclosure.  

This typically is investors.

But again, condo's sell on "comps" and apartments ( i know you listed a 3 unit in your example but regardless) typically trade at what they make.