Right now I heat my house with a Classic outdoor wood boiler. I built the house in 1990 so it is fairly well insulated. It is roughly 2600 sq feet with 100 linear feet of baseboard water heat. Looking at propane boilers. Local dealer has a Peerless L98-525 on a blowout for $1250. It is 120,000 btus. I was told the formula for sizing a boiler was 600 btu's per foot. He said this would be fine for me since my total is with the 100 feet of baseboard is 60,000 btu's. Anybody have any input ? 120,000 output seems low compared to what I see other people have. I think my Classic is around 250k output. But it is oversized since I wanted a bigger door and did not need this much. Any help would be appreciated since if this boiler is good I want to get that price.I also do not have any type of boiler now in the house to compare to.

I have radiators, not floorboards, but it's still hydronic heat. My 1980's Peerless boiler is rated at 150,000btu, and it does a good job heating my 3000sq/ft house, converted into a house in 1914, so not very well insulated. I would think 120,000btu would be fine for 2600sq/ft, but I'm not by any means an expert.

That doesn't seem logical to me.
The 600 BTU per foot thing makes no sense . There is every chance I misunderstand but........It seems to make more sense to me to make your calculations based on square footage or volume than it does to base it linear footage of the existing system

I believe your boiler is sized to the capacity of your emitters. Whether or not your emitters are properly sized to your house is a separate issue, but hopefully they're sized properly. Emitters are rated at a certain BTU at a certain temperature, cast iron radiators and floorboard btu ratings are typically given @ 180F I believe, meaning a floorboard heater emits 600btu per foot of length @180F. Obviously the water isn't always that hot (if your emitters are sized properly), so they aren't always emitting that much btus.

I did have the baseboard calculated when I built and my house is fine heat wise. I have heard that heating companies over size to be safe.Was told it is a good price for this boiler so if I need something bigger in after a year of use I would think I could sell it for $700 or so and not be out too much.

Is it going to be fairly easy to plumb in? I moved my boiler over about 3 feet in my basement so it wouldn't block the bottom of the stairs, and just moving it three feet was a nightmare-moving pipes, running electrical, moving gas pipe, running new vent pipe, etc etc. I'd imagine going from an outdoor unit to an indoor would be a pain, but I don't know anything about the oudoor units and how they're plumbed.

I'm not a heating guy but I am a retired engineer and can do numbers. There are numerous websites that will give you guidance. Google "residential heating btu per square foot." At least one says a general rule is 20 btu per square foot. That would put you needing 52,000 btu. The 100 linear ft of baseboard you have @ 600 btu/ ft would, as you noted, indicate 60,000 btu is needed.
That's a reasonable match. I would suggest you ask what the output water temperature of the gas fired unit is and compare it with your wood boiler. If both are close, the 120K unit appears to be significantly over sized.

If you need 60 thousand BTU to heat your house then a boiler with 120 thousand BTU is fine. My house requires 30 thousand BTU for heat and my boiler is 80 thousand BTU out put. Do you use the boiler to heat your domestic hot water? If so then you have to add the indirect water heater BTU needs in as well. We always used a minimum of 30 thousand BTU per one thousand square feet as a general rule without doing a heat loss report.

No, I have an electric hot water heater which I like.