What am I looking for? All I know is my old boiler was 140k btu and natural draft, and it is currently very dangerous to operate. I don't have an opinion on the features you mention since I don't understand any of the terms without googling them first
You may not need a 140k btu boiler.
Oversizing causes short cycling in theory; if u have too much capacity relative to the rads installed the burners will cycle on and off a fair amount during extended heating cycles. The aqua-stat cycles burners on and off to maintain water temperature, prevent overheating even during a continuous call for heat.
Short cycling reduces efficiency. It takes a good 10 to 15 minutes for a forced air furnace to reach steady state efficiency/capacity, and with the boilers, probably longer.
You can buy a natural draft boiler last time I checked but with a damper and spark ignition, makes it a bit more efficient than what u have but still not efficient. I don't recommend u buy one.
Induced draft is mid efficiency, exhaust vented by a fan. Uses less indoor air for combustion, higher steady state combustion efficiency - possibly due to less excess air, better control over the draft.
Condensing is 90%+ efficient but to hit 90%+ the water temperature has to be low. If the aqua-stat is set to 180f or something the condensing boiler won't save much compared to induced draft. To save much with a high efficiency boiler you must get an outdoor reset which lowers the water temperature as it gets warmer outside. The lower the water temperature the better the heat transfer.
Modulating uses a variable capacity gas valve so the burners don't cycle on and off to maintain the aqua-stat setpoint.
Get model numbers in quotes, post and the guys here who know boilers can advise.
i imagine the quote should include details on plumbing changes, expansion tank, etc, etc.
Boilers ain't cheap, i get that but saving money up front will cost u down the road.