Mitsubishi, Fujitsu, and Daikin together own something like 3/4 of the market in the US, with Mitsubishi the clear sales leader (if not necessarily the efficiency or innovation leader.) If you stick with any of those three you will probably have multiple levels of technical support & expertise. What you're looking for is a good HVAC contractor to advise on ductless mini-splits- there are many good mini-splits, not quite as many good HVAC contractors.
The competence and thoroughness of the installers will vary by quite a bit. I've grown to be highly skeptical of heat load estimates of those in the HVAC trades, and would either do my own or hire an energy nerd who has the right tools & knowledge before embarking on any new system. Oil use numbers can usually put a number on the whole-house load, but room-by-room load numbers are necessary in order to optimize the mini-split head (or heads) location(s) and size(s). All good heating systems start with a careful load analysis.
To sketch out a heat load for each room, you'd need to know how many square feet of exterior wall area, window area, the wall construction type & insulation values, the window type/U-factors, upper story ceiling & attic R, etc. We'd also need to know the exact location (zip code is good enough) to divine an outside design temp. This can all be done on a spreadsheet tool in a fairly straightforward manner. With the construction type & R values we could estimate the U-factors of the walls and ceiling, foundation etc, and throw in some air-leakage fudge factors- it doesn't have to be super-precise for sizing mini-splits but you don't want to end up oversizing by more than 50%, nor do you want be undersizing it for the spaces directly served.
Looking back to my quick & dirty analysis of your fuel use from a coupla years ago it looks like you have a 1400' house in a location with about a 10F outside design temp, and a heat load at 10F of something like 20-25,000BTU/hr (credible, but not etched in stone- do the room-by-room calc), which is approximately the output of a 1.5 tons of mini-split at +10F. If the floor plan is fairly open you may opt for a single head, and use cove heaters for balancing any critical doored-off rooms that wouldn't convect freely to the main space, but if it's a couple of large spaces it might be better to use two, maybe a pair of 3/4 ton units or heads (depending on whether the refrigerant lines to a 2-head multi route cleanly, or whether it's easier/better to use separate compressors), or a 1-ton + 3/4 ton, etc. This is not something well suited to design-by-web-forum, but if you take the time to do your own room-by-room heat load, and sketch up a floor plan, maybe. It's a worthwhile exercise even if you ultimately hire somebody, since you'll then have a better handle on what it really takes, and whether they're actually giving reasonable service. A single 1.5 ton name-brand mini-split runs $4-4.5K installed in my neighborhood. A pair of 3/4 ton or a 3/4 + 1 ton would run about 5-6 grand, as would a 1.5 to 2-ton 2-head multi split. YMMV. The
hardware is actually pretty
cheap, and most of the installation can be DIY, if you read up on it. Pay attention to "lowest operating temperature" in the specs- some go much lower than others. Most are still running at temps lower than specified, but with unrated/unspecified output & efficiency at lower temps. (The Mitsu GExx don't go as cold as the FExx, the Fuji RLX don't go as cold as RLS2, which doesn't go as low as the RLS2-H, etc.) Think seriously about your lowest temps of the past decade, as well as the 99% outside design temp when selecting a mini-split if it's going to be your primary source of heat.
The more rooms you can heat with the mini-split, the better, since it averages over 3x the raw efficiency of cove heaters or baseboards. Cove heaters in the intermittent-use rooms that use occupancy sensors to automatically turn off when unoccupied work out pretty well for many, and uses far less power than just keeping the room at a comfortable temp with a thermostat-only approach to control. The rooms run colder, except when you're using them, and colder==lower heat loss, and heat conducted thorough the walls from the mini-split rooms will limit just HOW cold they get. It's often a good strategy to slightly overheat the space with the mini-split to ease comfort issues in doored off rooms, but with cove heaters reasonably sized, the fact that they're warming you directly even when the room is still way below the thermostat setpoint makes it reasonably comfortable- it's sort of like an "instant on" radiator- radiating heat at you within 15 seconds of entering the cool room- it might be only 55-60F, but it feels warmer while it's running- like standing in the sun on a cold winter day. Like anything else, the cove heaters have to be sized reasonably for their heat loads. There's little energy-use penalty for oversizing them, but at some level of oversizing makes you feel like you're in a broiler.
A common installation error in the northeast is to mount the exterior units where they can be buried in roof avalanches, crushed by falling ice dams, or choked by drifting snow. Bracket mounting it on an exterior wall protected by overhanging eaves or rake of the roof- under the rake is better, since cornice-fall & roof avalanches are pretty much eliminated) is the right approach, or at the very least an open shed roof extending at least a couple of feet on the sides and front of the exterior unit. (I know a guy who had do dig up a freshly installed Daikin twice this past season- despite being ground mounted under the eaves, with a too-small protective open-shed roof over it to boot.) It's not just an air conditioner- it needs to keep working during and after blizzards.