Doing a downdraft makes zero sense when you already had a far superior overhead vent. Downdrafts are useless. Get an overhead hood and connect that to the vent above, as it was before. I’d have some serious reservations about the qualifications of your designer.

OK, 180˚F water temperature, 20-yr old boiler, 150-yr old house already reveals quite a lot. You probably have a mix of everything from cast iron radiators to copper tube aluminum plate base board emitters. Knowing how the piping from the boiler is configured will at least in part determine the water temperature that will reach the new basement emitters. If it’s a series configuration such that the basement emitters receive the hot water last before returning to the boiler, the water temperature will be at its lowest. If your system is piped that all emitters are in parallel, then they all get about the same water temperature whether the emitter is first or last. Even if the water temperature has dropped because its a series piping that still can be OK if the new basement emitters (fan coils) are sized for a lower water temperature with the correct water flow rate emitting the designed amount of Btu. I understand that financial resources might be limited. I would suggest consulting with a professional that is at the top of their hydronic game and understand all the various piping configurations not only for the basement Reno that you’re planning now but for future upgrades down the road as well. One of the most important things to know is that regardless the source of hydronic water heating (electric, oil, gas) is that the lower the water temperature your system requires - the more money you will save and the more efficient the system will be. Water temperature matters; piping, pumps, zone valves and controls all matter. The other thing to be aware of for the future is that air-to-water hydronic heat pumps are here, they are operationally the least expensive way to hydronically heat and cool, except for geothermal - if you can get you supply water temperature and system to function well down at about 120˚F and below. Hydronic heat pumps can also supply full capacity hot water in many cases. Future renovations involving air-to-water hydronic heat pumps could also supply full central hydronic cooling with zoning and dehumidification - without ever having to install any ductwork. This is a great option for heritage homes where you want to preserve the original look an d feel of the home as well as its heritage value. The link below for reference, is to but 1-type of system, with every passing year they are coming on-line to market - fast ! Taco System M IMPO SR

Basements below grade have small heat loads and almost zero cooling loads. Some of the heat loss from the current boiler and furance may be enough to keep the basement comfortable. If you install more efficient equipment if may change the basement's ambient temperature. I don't think you need to spend a lot of money to heat the fihished area. You could add electric baseboard heating to add some heat on very cold nights. Even a portable space heater may be sufficient for your needs. Another option is to use the combi boiler with hydronic baseboard radiators. It will cost more but the operating costs will be lower.

Just heard back...not including labour, almost $7,500. Yikes! I'm thinking I could heat the basement for quite some time on that! I also just relayed this to my energy advisor and he said that by dropping 2 or 3 ducts down & heating my basement to 15C-ish (59F) as recommended by the HVAC company, I will actually increase my energy score by a point or 2, but said it will come down to comfort as the floor won't be as warm. (he also reminded me to make sure that I can control the amount of heat that goes down so that I'm not heating it at the same temp as the living space) He suggested I try it for a winter and see how it goes & that if I decide that I'd like to insulate, I could even go to as low as R14(?) to keep costs down and even that would make a difference. So, for this first year at least, I'll go with heating the basement at a lower temp, not insulating the floor or painting the drywall, and see how it goes. I'll still need to install a dehumidifier as we have very humid summers here.