Hydronic baseboard vs hot water heating

[Marc Hauser]

I have a new three unit rental property, I'm currently in unit 1.  Unit 2  and 3's heating units either need to be fixed or upgraded.  They are both forced air.  There's issues with open vents that could cause co to flood the apartment, as well as gas pipes not properly placed. 

So, I'm considering replacing them altogether.  I'm wondering what is the cheapest way for me?  And cheapest way for the future tenants?  And what is the most effective?

I'm used to, forgive my lack of technical terminology, full house hot water heating with a boiler.  My last apartment had a 100 year old unit and every winter the super had to go into the basement every few weeks and bang on it to get it to work.  It also had a few disasters where steam flooded apartments because of corroded valves.

I'm considering doing it that way.  With the whole building using a single boiler paid for by me.  I'm wondering if aforementioned case is applicable to newer units?  How much would it cost, in terms of monthly bill?  What is the install cost?  Obviously, this would be a very rough estimate.  There is some website that gives that info, but I forgot it.

Hydronic baseboard sounds interesting because you can use oil which I assume is designed to have a lower freezing point than water, and therefor less likely to bust when it gets to -10 degrees.  The negative, I assume, is that the cost to the tenant might be high and they would have trouble paying the rent.

TL;DR: What are the pros and cons of hydronic baseboard versus a boiler and radiators that would heat the entire house?