Basement cooling loads are a tiny fraction of the cooling of the upper floors, since they typically have limited window area, and ~75F conditioned space above it rather than a 130F attic. But if you DO put a single-head mini-split in the basement, the 3/4 ton Daikin Quaternity is probably the best choice, since it has independently settable relative humidity & temperature setpoints, and can dehumidify in either heating or cooling mode (without any sensible heating or cooling if the temperature setpoint has already been met.) It's considerably more expensive than a standard name-brand 3/4 mini-split, but since the primary cooling load of the basement is latent-load, it's the right mini-split for the load.
A ceiling or mini-ducted cassette or two on the main floor could probably work out fairly well for both heating and cooling if you wanted to. A single mini-ducted cassette with split output in the top of the closet could probably serve both the office and bedroom. The big open dining/living room area could be served with a ceiling cassette. But that's a bigger investment than a cheap minimum efficiency no-name 1-ton with a wall-wart blob. A small 1.5-2-ton ducted split system would likely be the cheaper option for cooling-only, and would be small enough to provide reasonable latent cooling comfort (which is likely to be at least half your average load.) Like heating systems, most cooling systems are way oversized in the northeast, and while there's less of an efficiency hit to oversized air conditioning, you pay a penalty in humidity-comfort with oversizing. Mini-splits modulate with the load, and will usually provide higher comfort (and lower noise.)
Most raised ranches have a gross thermal bridge and air leak point at the cantilevered overhangs. Dense-packing that with cellulose or a full cavity fill of open cell foam on those overhangs can help both aspects a lot. If there is room to add 2" of rigid EPS as a thermal break (EPS only, lest you create a moisture trap) for the cantilevered joists that's worth it in the long term, but not the highest priority. Stopping the air leakage at the cantilever is far more important- many were (wrongly) built with a ventilation gap between the sub-floor and batts (if insulated at all), and have no real air-barrier where the joist-bays cross the foundation.
A hydraulic separator would make for an awfully expensive junk filter if it's not really needed from a flow-isolation point of view. The primary purpose for hydraulic separation is to be able to set the radiation flows and boiler flows separately. The radiation in the system design will have a range of flows at which it will work well, the boiler has a hard minimum flow, but also a max delta-T at which it can be operated, and over pumping the boiler leads to erosion on the internal plumbing. It's not uncommon to find a satisfactory flow rate at which both are happy, but this is best left up to a competent hydronic designer who will stand behind the design rather than a taking web-forum napkin-math stab at playing Jr. Hydronic Engineer.
You don't necessarily need a $400 system component for the hydraulic separator though- when the flow requirements are modest it's possible to cobble up an acceptable hydraulic separator out of 2"-3" fittings with reducing tees- there's no real magic to it (but you still need to do the math.)
BTW: While our average winter temps are more severe in central MA than the interior northern NJ, the design temps are not dramatically different. My 99% design temp is +5F, only 5F cooler than the +10F 99% design temp for Patterson or New Brunswick, whereas the binned hourly average winter temps are about 8-10F cooler.