Ontario is a big province with a wide range of climate (eg. Kenora is a heluva lot colder than Mississauga, and Fort Severn is colder still!). Care to narrow it down a bit? The location is necessary to be able to estimate the 99th percentile temperature bin, and the average heating season outdoor temp, both are relevant to specifying a heating system.
Wintertime low temps of -20C isn't a very severe constraint (are you on the Ontario Riviera or something?), and -20F isn't bad either. A typical 2x4 framed 1000' house that's insulated would have a design heat load between 20,000-25,000 BTU/hr @ -20F.
Do you have access to last season's gas billing (exact meter reading dates & usage numbers).
Even the smallest Navien combi (the NCB-180) has minimum firing rates are too close to your likely heat load, which means it can't modulate with heat load much, and not enough burner for really decent domestic hot water performance at your likely incoming water temperatures. Combis are rarely a good fit, they're better for houses with really big heating loads, but modest hot water needs, which is the converse of how it is with most houses.
A better solution would be a fire tube mod-con with a big turn-down ratio and a indirect fired hot water tank (or a stand-alone tank with it's own burner.) You may or may not have enough heat load to even run the smallest mod-con or combi in condensing mode without short cycling even the smallest boiler, but I suspect it would be pretty bad with a Navien combi.
How much radiator, or baseboard etc do you have?
What is the input and output BTU or wattage ratings of the current boiler?
The NTI Trinity TX51 (57K at max fire, 7.1K min- literally half the min-firing rate of the smallest Navien combi) is probably a reasonable fit for a boiler, or the Lochinvar CDN040 (40K max, 9K min). The L
aars Mascot MFTHW80 fire tube boiler is 80K max, but modulates down to 8K, and should be pretty cheap. Like Navien, it's made by a first-tier Korean boiler company (Kiturami). The exact same boiler is sold in the US for about
USD $1700 under the Westinghouse name plate, but Laars probably has a bigger presence in Ontario. The low pumping head of the fire tube heat exchangers and tolerance for high delta-Ts makes it a pretty cheap & easy swap out for a cast-iron boiler, since (unlike the Navien or Lochinvar) there is never a need for a hydaulic separation point (aka "primary /secondary" plumbing) - it can be pumped direct and function without a problem in 99% of the heating systems out there- practically DIY-able without a lot of hydronic design skills.
I suspect the TX51 would be fine pumped direct too, if that's somehow cheaper than the Mascot MFTHW80. Lochinvar is in the mode of voiding the warranty if it isn't plumbed primary/secondary, which adds the cost of another pump and more complexity to the installation. Navien insists on hydraulic separation too, and is happy to sell their pre-made manifold of closely spaced Tees with every boiler & combi, which is over & above the list price for the boiler itself.
Quotes for swapping out a cast iron boiler with something exactly the same size can be as cheap as $5K in my neighborhood, but it's usually a bit more. Mod-cons and condensing combis are usually closer to $10K. But swapping-in the Laars-badged 80K Kiturami (Mascot MFTHW80) should not be more expensive than a cast iron boiler, and could even be cheaper. The boiler itself isn't any more expensive than equivalent output cast iron, and the venting (being plastic) is cheaper, and even the plumbing can be simpler (no need for boiler bypass or other plumbing tricks to protect the boiler from condensing temperature water needed, since condensing temperature return water is the active goal.) The output at high fire is on the ridiculous overkill side for a house that size, but since it can modulate down to 8K-in/ 7.5K-out it's probably going to work in condensing mode even with minimal amounts of radiation to work with.