There's no positives to putting glycol in it unless it's some place that's used intermittently and would likely freeze up. There are many radiant space heating apps in NewEngland being driven by tank & tankless HW heaters. Tank heaters should go 15+ years in closed-loop heating apps- longer than in domestic hot water situations since it's not being constantly infused with corrosive fresh, oxygenated water. Keeping it it pressurized at ~10psi or above is sufficient to keep oxygen from infusing via the PEX.
A cast-iron boiler sized perfectly for the load is only modestly more efficient than a tank heater. The raw combustion efficiency of a standard HW heater is ~78-80%, compared to an atmospheric drafted cast-iron boiler's, ~82-84%, and extra plumbing is required in low-temp applications like radiant to protect the boiler & chimney from condensation and thermal shock, whereas the HW heater is just fine with 70F return water. In low & very-low heating load situations it can be hard to rationalize going with a standard cast iron boiler, let alone a 90%+ efficient condensing boiler at more than 5x the up front cost.
A cast iron boiler 3x oversized for the peak load will be LESS efficient than a hot water heater.
The size of the tank is fairly irrelevant in a slab-radiant system- the thermal mass of the slab itself is more than sufficient buffering. The size of the burner IS. Look up the burner rating of the one that died, and guesstimate backwards. If the think was running 6+ hour burn times on the coldest day of the year it's probably slightly undersized for the heat load. If it was never more than an hour or so, with at most a 50% duty cycle it's probably somewhat oversized, but not so much so that it's fallen off an efficiency cliff. If it runs well-over 50% duty cycle on really cold mornings, but keep up, you're golden.
If you have good fuel use and heating degree day data to correlate it with it's possible to calculate the heat load to a high degree of accuracy, but that may not be important here. If the old one kept up, a new one with a simliar sized burner will too. But if it was a 60KBTU burner and you could get a cheaper smaller tank with a 40KBTU burner that would handle the load as-efficiently or more, smaller is generally better, delivering more efficient spring/fall part-load performance.
If you're willing to pay a little bit more for efficiency, a direct-vent hot water heater (that takes it's combustion air from outdoors, eg: AO Smith ProMax GDV series) is typically good for at least 5% fuel savings in a heating application, in which case you'll have beat an atmospheric drafted cast iron boiler on as-used performance. (I considered one of these for heating my place, but went with a more expensive direct-vent tankless as a boiler, with an indirect-fired HW heater as a system buffer. I don't have the thermal mass or low heating water temps of a radiant slab- you'll likely get better performance out of it than I would have.)
The worst efficiency you'll ever get out of is during the fall/spring seasons, where it might fall down to 70% (which is about where your 83% AFUE cast iron would be getting under those conditions) but unless you keep it hot all the time in the late spring/early fall "just in case" you might need a bit of heat, it'll beat it's EF numbers by double-digit percentages. (A 0.60EF heater will deliver at least mid-70s under any real space heating load.) The best efficiency you'll ever get out of it is just below it's raw combustion-efficiency, typically ~80%.
