Hi Pygargus,
normally you don’t have to much selection in Januar, i went to Istria for a week last year at Easter many think was still in the winter sleep, but we manage to find nice apartment few km. from center of Porec (with fireplace in the apartment), in the ground floor the owner was running a very nice restaurant, which looks like agro museum inside, the food there was excellent so if you are interested you can find more information here on Istria page.
The weather can be quite tricky this time a year, you can have nice sunny days, but also days with lots of rain.