CPVC is the most commonly used vent material for condensing boilers in my area. Other types of PVC are not allowed as vent piping when routed up a chimney chase, but CPVC is fine.
A standalone gas hot water heater typically has a 32,000-45,000 BTU (input) burner. Even the smaller condensing gas boilers have higher burner output. With indirect fired water there is no need to increase the boiler size, but for smaller boilers it's best to operate the indirect as the "priority zone", which suppresses the zone calls from space heating thermostats until the tank's aquastat is satisfied. (Being able to assign priority to a zone is a standard feature on nearly all modern zone controllers. ) The recovery time of a 50 gallon tank from a totally-tepid 30C up to fully scald-worthy 55C with 40,0o0 BTU/hr of heat coming from the boiler is about 28 minutes. Most houses won't fall very much in temperature in that short of time even at -20C outdoor temperatures.
A single standard flow shower takes about 75,000 BTU/hr in the middle of winter, but it's only used in bursts of 0.1-0.2 hours at time, which is why you store the heat in a tank, or why you need a really HUGE burner on a tankless hot water heater. You need to cover other hot water draws that might occur when somebody is in the shower to avoid the screaming match. Filling tubs from a tank will be an instantaneous load more than 100,000 BTU/hr, often over 150,000 BTU/hr, but it's only for a short period of time.