Are the windows double pane or single? Double pane windows provide more insulation. What is the flooring material in the room with southern exposure? They provide the thermal sink needed for stable indoor temperature, and preferably are made of stone or concrete. Feel the floor with your hands, if it is cold in the afternoon, it is probably not a sufficient heat sink.
Wall and roof insulation is the most important thing, but can be hard to assess. I would feel the walls when the interior is heated above the ambient temperature. If there is a lot of cold spots near the ceiling, windows, doors or corners, that suggests a poor insulation job. With brick exterior, the only way to fix it is with blown-in insulation (much easier if you have access to an unfinished attic) or taking down drywall and adding batting (expensive and time-consuming). For roof insulation, if there is attic access it is straight-forward to assess and add more as needed. If you are very serious about passive solar, I would pass on the house if either of these are poorly insulated as it is difficult to remedy in an cost-efficient manner.
The best way to objectively assess would be finding information on the previous years' heating bills and ask the owners how warm they kept the place. If the owners are unwilling to provide that, move on.
If you do decide to purchase and live there for a long time, I would recommend planting trees on the sides other than south. Consider fast-growing trees that will get to 10 feet in a few years.
With regards to the heating bill, the house is small enough to consider a wood pellet stove for heating the areas you will spend the most time. Using stone for the surround is a good way to add a thermal sink to the living area, especially if it faces the southern windows. The cost may significantly depend on local building codes.
Good luck! Check out your local library for more books on passive solar heating!