Number #2 oil has approximately 138,000 - 140,000 BTU's per gallon
A cubic foot of natural gas 900. 1,200 BTU's
You need special venting for an indirect heater then who is going to service it?
Before anyone can give you a decent answer they would have to know your demands such as how many people are going to use the HW, how many bathrooms do you have return circulation what is the flow rate requirements? Is this also for mixed used (commercial) what recovery rate do you need?
There is a lot of information needed to give an educated answer