"Dry" means "dry-bulb temperature", which is the outdoor air temp. "Wet-bulb" temperature is a measure of the absolute-humidity, and irrelevant from a heat load point of view, but important for cooling load calculations. The 99% design temperature is the 99th percentile temp from compiled binned-hourly temperature data over a coupla decades. While 1% of the heating season hours in a decade might be below that temp, even if your heating system had EXACTLY the output that would keep up at that temp, you won't get cold, since it never stays below that temp for more than a few hours at a time, and those hours usually occur while you're in bed (unless you're an up at 4AM dairy farmer.)
The typical or average base 65F heating degree-days in most of SD is ~7500HDD, give or take. Given that it was a relatively mild winter I'll do the example-math on 7000HDD. (see:
http://www.builditsolar.com/References/hddmap1.jpg )
In the absence of an input/output boiler nameplate info I'll assume it's steady-state efficiency is 85% (it may in fact be lower), so with water heating & other uses this will be a clear overestimate of the heat load at -5F.
1000gallons in 7000 HDD is (1000/7000=) 0.143 gallons per degree-day.
A gallon of propane has about 91,600 BTUs, so that's (91,600 x 0.143=) 13,100 source-fuel BTUs per degree-day
Burned in an 85% boiler means only (13,100 x 0.85=) 11,135 BTU per degree day ended up inside the house- the rest went up the flue.
There are 24 hours in a day, to that means for every degree-HOUR it took (11,135/24=) 464 BTU per degree-hour.
With a base balance point of 65F, the heating-degrees at an outside design temp of -5F are (65- -5 =) 70 heating degrees.
So your heat load at design condition is less than (70F x 464=) 32,480 BTU/hr.
If you were using deep overnight setbacks etc you might have saved ~10%, but that's likely to be more than offset by hot water usage etc.
If the nameplate efficiency is 80% rather than 85% (divide the DOE BTU-out by BTU-in for the steady state efficiency), multiply that 32.5K number by the ratio: 80/85 x 32,480 = 30, 570 BTU hr.
That's the range you're looking at. Don't be surprise if the name-plate output is 2, 3, even 4x above that.
Even a smallest-in-class 50-60KBTU/hr-input mod-con can deliver that kind of heat with room to spare and would still modulate decently. The amount of condensing efficiency you'd get out of it would depend on how many feet of baseboard you have. If it's cut up into zones you may have to add mass or combine them to keep the boiler from short-cycling at low output temp (lower temp == higher condensing efficiency), but you'd be looking at at LEAST 90% average efficiency, and if you tweaked the temperature curves for the outdoor reset control carefully you could hit 95%.
If you think $2 propane will continue to be available (I've not seen it that cheap locally for a decade or so) and want to go with a cheaper boiler, there are some $1600-2000 mid-efficiency boilers out there with output ranges that aren't ridiculously oversized. (eg the smallest
Lochinvar Solution or the
Burnham P203-LP But a
Solo-60 or
PF-50, would only set you back about a grand more, and use 15% less fuel. So even at $2/gallon and 850 gallons rather than 1000 gallons/year the simple-payback is at most 5 years, and if propane goes up it'll be even quicker.
Even a $2/gallon @ 95% efficiency it's still a a bit cheaper to heat with a ductless heat pump even with 15 cent /kwh electricity. In SD the average seasonal coefficient of performance would be between 2.0-2.5. Assuming it's only 2.0, for every kwh you get (3412 x 2.0=) 6824 BTU of heat into the house. With propane at 95% for every gallon you get (91,600 x .95= ) 87,020BTU into the house. So it takes (87,020/6824=) 12.75 kwh to deliver the same amount of heat as a gallon of propane, but at 15 cents/kwh it's like heating with (12.75 x $0.15 =)$1.91/ gallon propane in a high-efficiency boiler. But whereas propane prices are highly volatile, electricity prices are regulated. During the shoulder seasons with temps in the mid to high 30s or warmer a ductless would deliver heat with a COP of 3.5-4.0, at which point it's about half the cost of heating with propane. If your electricity prices are 10 cents it almost a no-brainer to drop in a 1.5-2 ton high-efficiency ductless ($4-5K) since it would cover the load for all but the coldest of days/nights. When it's under +5F outside the COP drops to about 1.5, and heating with $2 propane would be cheaper until it warms back up to a balmy +5F.