I have an LG minisplit system in my 1885 house, works fine, no ducts. Mitsubishi makes a good one too, www.mrslim.com

Hello:
I am renovating an office that is in my garage. The space is about the size of a parking space (sorry - don't have measurements in hand). I'm a professor and use it only sporadically - most days in the summer, most days in January, and on and off the rest of the year, depending on what I'm working on. It currently has an ancient wall AC unit that barely works (though the brick garage stays quite cool in the summer and there are big french doors to the garden so I don't use it much anyway). It has ancient baseboard heaters some of which are hidden behind shelving units and cost a fortune to run (my January electric bill is the highest of the year). My contractor has recommended a mini split unit instead of a new AC unit and new, relocated baseboard heaters. My question is: is this worthwhile even if I don't use the AC very much? Also, I have read that the heat pump aspect of the minisplit unit doesn't really work in sub-freezing temperatures. January and February are exactly when I most need heat and in Washington DC it is certainly below freezing during those months. In other words, my main need is for heat not AC - it can be REALLY cold out there on a January morning! does it still make sense for me to install a minisplit? what do I need to know to get one properly installed?
Many thanks
Jess