Lol they really started calling portable ACs “micro heat pumps”. For fuck’s sake, a heat pump is just an AC with a different valve, invented ages ago, running in reverse!
Air based ACs/heat pumps can only work in a limited range. Modern coolant is quite effective at getting higher ranges, but for 0F you may need something like a heat pump dug into the ground to heat effectively. Still, for most days, these “micro heat pumps” should work pretty well.
Like with portable ACs (as they are the same devices), look for models with two hoses. The models with one single hose will pump out cold air, creating negative pressure inside your house that will suck in cold air from outside again. With a two hose solution, the air that gets cooled down/gets the heat sucked out of will be pumped back out immediately. For some cheap units with a single hose you may be able to convert them into a double hose design with 3D printing, duct tape, and messing around, but you should check if that’s possible before purchasing such a unit.
Don’t fall for those Amazon scams that sell amazing coolers/heaters for just a few bucks. They’re usually tiny, and at most produce a warm/cold breeze to keep your hands or face cold/warm. If you’re going with resistive heat (easy, cheap, portable, but half as effective as a heat pump in most scenarios), get one of those big units, not the Amazon shit that’ll burn your house down.
If you haven’t already, invest in keeping the heat inside as well. Door strips and better window glass can save a whole lot more heat than a portable heater will give you if your house isn’t insulated well. Find out where the drafts are and get rid of them (don’t get rid of ventilation all together, though, obviously).
If you need to selectively heat your home, keep an eye on your piping. Turning off the heat in a few rooms may save you money, but if your pipes freeze and your house floods after they burst, you’ll be spending every cent you’ve saved on repairing damages.
As always with electric heat, watch out for overloading your sockets. Resistive heat will easily pull a couple thousand watts out of the wall, and if you’re not using 220V sockets, that may cause the wires inside your wall to heat up (but not in the good way) or even melt. All heaters worth their salt are high-powered electrical devices that should not be plugged into sockets or power strips that aren’t rated for the total load these devices and anything else plugged into the same circuit may induce. The same goes for plugging high wattage devices into smart plugs (which you could use to get something like a thermostat effect going): make sure they’re rated for the wattage you’re subjecting them to. Oh, and please use a grounded socket, preferably one protected by a GFCI.
Read the manual of whatever heater you end up buying. They’re not very interesting, but they usually come with at least one “I didn’t know I wasn’t supposed to do that” safety warning that could save your life.