We have previously got great value from the Caledonian Hotel (£25) in January, because it normally caters for huge bus parties, and had just decided to stay open and tick over that year, but it is coming up as FULL on Laterooms, (I assume closed). If you are prepared to spend the outrageous sum of £45, the Morefield Motel is open...don't know your dates, so just tried the 12th.. We stayed there when I completed the Grahams on Stac Pollaidh this summer, and it is fairly basic as hotels go, but has warm showers, clean beds and a bar on the northern outskirts of Ullapool. See
https://www.laterooms.com/en/p4650/Reservation.aspx?hid=268217