Consider this another vote for installing a conversion burner w/ indirect hot water heater and smarter heat-purging controls. Odds are pretty good you can get this done for under $3K, maybe even under $2.5K.
An indirect hot water heater is a tank with an internal heat exchanger that allows the boiler to heat up the volume of potable water:
1 Hot water pipe (to taps)
2 Heat trap
3 Cold water inlet
4 Hot tap water storage tank
5 Heat exchanger (filled with boiler water)
6 Stored hot tap water
7 Pump
8 Hydronic heating pipe (to house radiators)
9 Boiler for house heat
----------------------------
The "smart" contollers like the Taco PC700 or Intellicon 3250 HW+ or Becktt Heat Manager improve the system efficiency by keeping the boiler at a lower temp during standby mode, but they have somewhat different approaches/algorithms by which that is achieved. The Taco senses the outdoor temp and uses that to determine the appropriate average boiler temp, whereas the Intellicon (& Beckett, which is an Intellicon design) "learns" to anticipate the end of a call for heat based on recent burn cycles, and turns off the burner early, purging heat from the boiler into the system to finish off the call for heat, an purges it further down to the (user programmed) low-temperature limit at the beginning of a call for heat.
When the boiler is grossly oversized for the load (as yours almost certainly is) the outdoor temperature hardly matters, since even at the boiler's safe low temperature limit it will always be hot enough to heat the house. The Weil-Mclain P-WGTO-3 has an output of 100,000BTU/hr, and for a 1600' house that would come in at a very high 60+ BTU per foot of conditioned space. Assuming it's a house and not a tent, that's probably about 400%-500% of the heat rate you'd ever need to keep the house warm, even at the 99% outside design temp (somewhere between +2F and +15F, depending on where you are in CT.) When it's that oversized, you're probably slightly better off with the Intellicon. Were it less than 200% oversized for the load there would be a comfort benefit to the Taco.
While burning oil the lowest temp you can run the boiler without running into corrosive condensation problems is ~140F, but once you've converted to gas you can run it as low as 130F without issues. Running it at the lowest temperature possible has an efficiency benefit from both a standby loss and distribution loss point of view. The smart controllers allow you to run it at lowest possible average temperature without efficiency robbing short-cycling, and without destructive exhaust condensation. At 4-5x oversizing you can expect to get a 15%+ reduction in fuel use by using heat purging smart controls. Newer-better boilers incorporate these types of smart control, as well as internal plumbing to run the system at even lower temperatures (for lower distribution losses).
If you replace the boiler, it's important to determine the true heat load, since oversizing it costs more up front, runs less efficiently and higher maintenence & comfort issues. Most 1600' houses in CT will have a design condition heat load of under 30,000 BTU/hr, and many will be around or under 20,000 BTU/hr. Both the Rinnai & Navien combi-units are on the big side these types of heat loads, and may short cycle unless you have high-mass radiation. If you have a mid-winter oil bill with a "K-factor" stamped on it and a zip code (for design temp & weather data) it's pretty easy to calculate a hard upper-bound on where your heat load lies.