What is youranticipated design-day heat load, and how many heating degree-days to your season? Square footage doesn't tell much, without the proper context. Got a zip code? (From which we can at least estimate the HDD number.)

Heating boilers are usually big enough to support substantial domestic hot water loads in colder areas. In very temperate climes or very high DHW looads you may need a very large indirect-fired tank if you're needing to fill a 100 gallon soaking tub or running 3 high-flow showers. But in most instances it's cheaper/better to have one burner for both, even if your heating loads are low (as they likely are for you, but we don't actually know that...) Peak HW loads are typically several times the peak heating loads, and oversizing the heating boiler just to handle the HW load tends to reduce the efficiency in heating mode. The proper compromise between burn size & storage capacity needs to be designed in at the outset.

Under no circumstances should you run an "open" system, that uses potable water for heating purposes (due to the stagnation bio-hazards.) Heat exchangers are cheap compared to even one short hostpital stay, eh? The heating water will have less corrosion & maintenance issues if it's not constantly taking in fresh water too (dissolved oxygen & grit eats up pumps & valves.)

There are many good modulating condensing boilers out there, but none are particularly cheap. Even the smallest in any manufacturer's line run ~$2800-3K. Depending on your annual fuel use it can be hard to rationalize high-efficiency boilers for very light loads. Any boiler can be made to work, but it's sometimes easier & more efficient to design a tankless-combine heat/hw system than going with a higher-mass cast-iron boiler. The thermal mass in the heat exchanger in a tankless is similar to that of a high-efficiency boiler, and they're more tolerant of the cooler return-water temps of radiant than cast iron (which can still be used, with a bit of plumbing to protect the boiler by mixing in some output with the water from the radiant before it enters the boiler.) Takagi's entire lineup can be used in combi-applications, but most other manufacturers restrict it to particular models. If your loads are low enough you can even use a tank HW heater as a combi-system, which is likely the cheapest way out when it works.

I'm a bit surprised when I see radiant in SoCal, since a hydronic coil in the combined heating/AC air handler can deliver about the same efficiency as radiant, with sub 100F return water. Hydro-air doesn't have the same cushy warmth underfoot you get in colder areas with moderately insulated houses, but in high-R envelopes & very-light heating loads the floor temps required to meet the heating load just aren't that toasty-warm anyway. Hydro-air combis using tankless heaters or mod-con boilers are becoming more common in your area as well as FL, but they've even been done with tankless & tank water heaters in Canada: http://dsp-psd.pwgsc.gc.ca/collection_2007/cmhc-schl/nh18-22/NH18-22-106-108E.pdf