Advice for replacing water tank and boiler

[icu222much]

I am looking to replace my rusting gas hot water tank (15 years old) and leaking gas boiler (30 years old), and am trying to determine what the most cost-efficient long-term solution (30+ years) would be (ie - installation fee, maintenance cost, gas bill...).
A bit about my house:

• 3300 sqft 30 year old detached (5 bed, 5 bath)
• Vancouver, Canada
• ~$13/GJ gas price ($300 per winter month, $60 per summer month)
• 5 zone radiant heat on both floors
• Poly-b pipes

Questions:

• From my research, there are three systems I can use. Are there any other systems/options I have missed?
  1. Standard efficiency hot water tank & standard efficiency boiler
  2. High efficiency hot water tank (aka hot water on demand) & high efficiency boiler
  3. Combi system high efficiency with a storage tank (I was told I would need a water storage tank with a home larger then 3000sqft)
• Is one type of system (standard efficiency, high efficiency, combi system) more reliable then another? I figured the high efficiency and combi systems are more complex, therefore can break more easily.
• Would the maintenance for each system be similar? What should be done to maintain the system, the frequency, and the cost?
• Generally, how long does each type of system last?
• What do you think the gas savings would be with each type of system? From what I understand, a standard efficiency system is roughly 80% efficient, where-as a high efficiency would be around 95%. In that case, would I be saving just 15% on my gas bill if I purchase a high efficiency system?
• I have a few installers coming next week. Are there any specific questions I should ask them?

 

[Fitter30]

Heres one article about poly b pipe its history and problems.
https://www.poly-b.com/
What water temperature did the old boiler run at and the btu rating? All condensing boilers are 95% efficient because of the low return water temperature of 130*f or lower. Above 130* 86%. Has the existing domestic hot water kept up with your demands? Does the boiler cycle when it gets to the extreme cold? Take a pic of the boiler and water heater tank name plates.

 

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[Myhouse]

Since you have a boiler, ask them about indirect water heaters. No vapor ignition safety nonsense to worry about, which direct-fire water heaters require (in the USA anyhow). Plus, indirects have nearly zero standby heat loss.