Unless the thing is completely toast, replacing the boiler isn't a very cost-effective efficiency improvement. You can probably get double-digit improvements out of a better controller such as the Intellicon HW+ (worth doing, if the boiler has another 5-10 years in it, which it might), but dropping money into a condensing propane or high efficiency oil boiler you're looking at a 25-30% savings, best case.
A more cost effective solution is to
keep the boiler (but adding an intellicon) but to add an R410A refrigerant mini-split heat pump with at least 2-ton (24,000BTU/hr) nominal heating capacity which would cut down your heating cost substantially if used as the primary heat source, using the boiler only as the backup during the bitterest weather. In an OH climate you can count on getting an average coefficient of performance (COP) of 2.5, and maybe close to 3.0. At 2.5, it means for every kilowatt-hour (kwh) of electricity expended, you get 2.5 kilowatt hours of heat put into the house.
The economics of it work this way:
1kwh=3412 BTU, so for every kwh used by the heat pump you get 2.5x 3412 = 8530 BTU into the house. For every 10kwh you get 85,300 BTUs into the house, which is about what you'd get out of one gallon of propane in a 94% AFUE condensing boiler (assuming you have enough baseboard to run at condensing temps most of the time.) If you're paying 12 cents/kwh, that's roughly equivalent to $1.20/gallon propane. Even at 20cents/kwh (probably way more than you're paying in OH) that would be $2 propane.
Priced propane lately?
Assuming your 30year old boiler is running 75% efficiency or better (which it probably can, with an Intellicon HW+ controller), call it 85%, every gallon of oil delivers 0.85 x 140000= 119,000 BTUs to the house. With a mini-split heat pump that would take 119,000/8530= 14kwh. At 12 cent/kwh running the heat pump is equivalent to $1.68/gallon oil in your boiler, at 20 cent electricity it's like $280 oil.
In a state of the art 90% Buderus you're looking at 126000 BTU/gallon, which is closer to 15 kwh in the mini split, so at 12 cent electricity it's like $1.80 oil in the Buderus, a 20 cents it's like $3 oil in a Buderus.
A 24,000BTU single-head mini split is under $5K. Add about $1200-1500 per head for the interior units for multi-splits. You can probably even put in a 36,000 BTU multi-split for less than the cost of state of the art Buderus, and it would probably cost half as much to run. There are multiple vendors (Fujistu, Mitsubishi, Daikin, Sanyo, LG, etc etc.), but finding one with good local distribution & support is a good idea. They're practically (but not completely) idiot-proof to install, but there's always a better idiot to be found, if you're really looking. You want to get something with an HSPF (heating season performance factor) of at least 9.5-9. I've been sticking with the
short list compiled by a pacific northwest monitoring program, but there are others out there.
During the shoulder seasons they'd run a COP of 3 or better, but below 10F they're only about 2-ish, at which point running the oil boiler isn't dramatically more expensive as the backup. Don't use set-backs with them- they run more efficiently if you "set and forget". Set the T-stats for the boiler a couple degrees below that of the mini-split and you're golden- it'll only fire up when the mini-split can't keep up, which will be mostly during the coldest 5% of winter hours.