Counting the total surface area of radiators IS the right way to figure out the sizing of a STEAM boiler, but it's a terrible methodology for sizing hydronic systems.

The math on source fuel BTUs is fairly straightforward. You're trying to figure out from gallons per year what how many BTUs per hour it takes to stay warm under "design conditions" (the outdoor temp about the 99th percentile coolest hours of the winter). In your neighborhood that temp is about +10F (in mine it's about 0F.)

The information you know is in gallons per year (~350)and the heating degree-days (HDD) per year, which ~5200, in your neighborhood. The heating-degree-days of any particular day is how many degrees the average outdoor temp was below 65F (the rough average break-even point for heating/cooling for most single-family homes.) Count them all up for a season, and in your neighborhood it'll be around 5200, give or take a few percent, rarely differing by as much as 10% in any single year.

From gallons/year and HDD/year you can calculate gallons/HDD.

Great, but you still want to get to BTU/hour, right?

Since there are 24 hours in a day you can calculate gallons per degree-hour. (That is, gallons per degree below 65F). So divide the gallons/HDD number by 24 hours and you get gallons/degree-hour.

To convert the gallons to BTUs, the generally accepted approximation for #2 heating oil is 138000/gallon, so multiply gallons/degree-hour by 138,000, and you have BTU/degree-hour.

Then to get to BTU/hour at +10F, multiply time the difference between 65F and 10F (55 degrees), and you'll have BTUs/hour of source-fuel consumed.

But in an 85% burner (most flame-retention burners of 20 years ago at least started out in that range, when properly adjusted), 15% of the source fuel energy went up the flue, and only 85% of the BTUs went into heating up the boiler & heating system water. So the actual heat load (the amount of heat necessary to keep the house warm) would be 0.85 times the source-fuel BTU/hour number.

You can pad it 5-10% if you use deep night-time setbacks, but since you're also heating hot water with it odds are the summertime & hot water heating BTUs that went down the drain far exceeds any setback savings. Take that 21,450 source BTU/hr number multiply by 0.85 and you're in the 18.25K range, and that is a firm upper bound. Being that the boiler is way oversized for the load and heating domestic hot water, 18.25KBTU/hr is probably at least 10% to the high side of reality, and may be more than 20% above what would actually keep you warm.

Which is the long-winded rehash of the conclusion that there aren't any truly right-sized oil boilers out there for heat loads that low. Some modulating condensing propane units can go about that low at lowest fire though. With oil ~55-60K (output) would be the very smallest, and probably not in a chimney-ventable unit. The 3-plate Buderus is 74K out, it's (relatively) cheap, and a reasonable compromise.

No matter what you install for an oil boiler, the bulk of the price is going to be the installation and system (re)design not the boiler. The difference in price between the smallest chimney-ventable Buderus and a Craigslist-scavenged boiler will be less than $1500, maybe $2K if you put in an oversized beast with an internal HW coil. A brand new G115WS3 is only a couple grand- add another $600-1K for an indirect HW heater, but then there's resizing the flue with the right-sized stainless liner (another grand), and a couple of plumber-days, maybe three by the time the system is purged and tweaked. Plus there are pumps/valve/fittings and small amount of design & tweaking of the bits of plumbing necessary to keep the return water temps in the non-destructive range and you'll be talking $6-8k anyway you cut it with a professional installation. Reselling the house with a brand-new brand-name boiler with a warranty may be easier than with a hacked-in-DIY craigslist-special of unknown history/longevity.

Insulating from the outside is possible, but it'll be expensive, not a short-termer's project under any circumstances.