How difficult is it to repair or replace a preserved wood foundation?
I am in Maine where climate is similar. There are very few Preserved or pressure treated foundations here. I have seen them on out buildings, barns and some summer camps. They are rarely used for dwellings, with good reason. Wood and water do not mix, regardless how well they have been treated. Even though some have been in used for over 50 years, every one I have seen during inspections has some degree of deterioration. Typical posts are 8X8 or larger and usually show a fair amount of decay at the ground line. Many have already been replaced with concrete sauna posts. Depending on the water and soil conditions, all wood in the ground is going to rot in time unless you are in the desert with no vermin eating away at it. To be honest, there is no way to accurately predict how long something like this will last.

As far as replacing them, the process is straight forward. The structure will need to be jacked up a bit and supported. The new foundation will need to be excavated and poured. How much you will need to raise the building depends on how large it is and how many points of support you need. The major factor is getting enough room to work under the structure. If only outside supports around the parameter, then you may only need to raise the building a few inches for a shallow block foundation, however if you are going to put in a frost wall, full foundation or 4 foot sauna tubes, you may need to raise and crib the building much higher to accommodate forms and machinery. This is really not a DIY project and should be tackled by a house rigger that has the tools and experience to do the work without destroying the house, windows, doors floors etc.   The basic utilities such as water, septic or sewer and electrical service usually have to be disconnected and modified as well. This can be a very expensive project.

If I were in you position and had the choice between two buildings, one on a solid concrete foundation and one on wood, the choice is easy. In your case, I would advise having a structural engineer inspect the property and render a specific opinion and possible replacement scenarios.