Opinions on DHW options

the combi is easier to install, the manufacturer baked in the complexity but the parts tend to be more proprietary if it breaks. the indirect the install is more complex but the parts tend to be off the shelf parts.

with the combi you're replacing the dhw when you replace the boiler, with the indirect they could end up failing at different times and stagger the replacements.

check your incoming cold water temp in winter before deciding on a tankless or combi. in my area 200,000 btu/hr in is more like 2-2.5 gpm because the incoming water is nearly frozen.(i'm doing the math again and coming up with around 4gpm, i don't know why i was thinking about 2 gpm before, i may be missing something)

you do need a big gas supply for a boiler and a tankless

maybe i was looking at something like rinnnai's table and it was doing something weird

The capacity for the tankless or combi comes from the capacity of the service too. Running a new line from the meter isn't a big deal, running a new service could be very expensive, or even just upsizing the meter can be expensive with some utilities.

The biggest load determines the size of the boiler. In some cases the DHW is a larger load than the heating.

With a large indirect, the boiler goes to priority for maybe 20 minutes or more to recovery.

The math is the same regardless of the type of heater 500 X flow X delta T

The selling feature, and sometimes downside of the tankless is the amount of BTU/ gas supply to make a lot of DHW instantly.

The other option is to reduce DHW demand. Low flow shower heads, reduce water pressure to 45-50 psi are two way to lower the DHW load, and cost of DHW generation.

If costs are a concern, plug in you actual numbers here.

Hard to beat operating cost of a HPWH or tankless.

https://www.efficiencymaine.com/at-home/water-heating-cost-comparison/

I'm not a huge fan of side vented gas fired appliances. It seems you or a close neighbor often end up smelling them, and some of the non variable speed WH types are quite noisy.

The mod cos, or condensing style tankless, with 2" pvc vents are not as much an issue if you can get the vent up high on a side wall.

I would do a Mod /Con with an indirect. You just can't beat the recovery times of an indirect water heater. A properly sized boiler matched to the indirect and you'll never run out of hot water. And don't forget the warranty. All the indirects that i have installed have come with a lifetime warranty on the tank to the original owner. So if you plan on staying awhile you have to factor that into the process. no additional venting. No additional gas lines. A simple sensor or aquastat to initiate a call for hot water. i have one and would never do any other.

you have to make sure the water quality, especially the chloride content, of your water doesn't void the warranty.

Where will you pull the intake air from? You do not want to recycle either the neighbor or your own exhaust gas. Does the neighbors exhaust and intake on the side wall?

Test you water for chlorides if you do go with a stainless tank or tankless. if you are on a public supply they should have a report like this available.

Here is a water quality spec for a stainless indirect. Over-softened water can drive chloride levels up. Chloride levels tend to be high in snow belt areas where the road deicers enter the aquifers.

I've found the glass lined steel tanks have the longest life expectancy, if anodes are maintained.

Bock, LAAR, Lochinvar offer this large diameter coil steel tank. The smooth coil works best in hard water areas also.

Every system is going to have its drawbacks. In my area indirects far outshine any type of system that's installed. On demands are high maintenance items and we change warranty heat exchangers all the time. Commercial water heaters like the cyclone? 3- 4 years top. Residential any style. one day past the warranty period. If i match up the numbers i probably replace 100 other style water heaters for every one indirect. chlorides or no chlorides. To me it's not even close