Or even electric-  a lot of electrics look cheesy, but I have one that I love (you can run it any time of year, with or without heat). 

Find the least obvious route to access the main living floors and cut wall,,,take out all bricks from that side,,,make it into a storage closet type thing and  put new heaters with duct work out sidewalls OR electric heaters with no ducts. 

I am doing a new roof,  Kitchen,  Full Bath, Flooring and some electrical.  

We are getting electrical and plumbing done in the next week.

Right now I'm focusing on my electrical and I need new service throughout the building.

Some have electrical contracts that can reach and exceed $100K.

@Giovanni IsaksenWe just acquired a 5 (soon to be 6) unit where there were window air-conditioners hanging from all units - despite the fact rarely gets over 85F here in the summer.Three of these units are 2-story, 2-bedroom ranging from ~1000 - 1200 ft^2, all have electric baseboard heat and all are on their own meters.

I bought a security system from Simplisafe.com, you don't need a telephone line, but do need electricity.

Foundation, roof, plumbing, electrical, and the HVAC.

You will find many different answers, but let me give you what i would do about your questions.1) I would hire a licensed person for anything major like plumbing, electrical, hvac etc.