There is an absolute obligation on the OC to repair and maintain the common property. However, that does not mean everything has to be in perfect, pristine, as good as the day it was built condition.
In the case of this fascia and gutter, the question I would ask is: Since it is acknowledged that the work needs to be done eventually, what is the risk that the overflowing gutter does further damage that ultimately makes the job bigger and more expensive than it would be if the work were done sooner? That someone is selling is irrelevant to a decision about whether this is a job that can be reasonably deferred for a few years or should be done promptly. If the job would be urgent without this person selling, then get on with it. If the job is not urgent, then don’t put up with being bullied if the OC has already made a plan and recorded its reasonable decision.
Since the matter has been discussed at AGMs, I assume the decision to anticipate doing this work in a few years time was minuted, ideally with something recorded about why the job was not urgent but nonetheless could be reasonably anticipated in the next few years. Even if it was not a resolution, if the minutes record discussion of an anticipated cost of around $30,000 in a few years time for this anticipated work, then nothing is hidden. The seller can say that the OC is aware of this particular bit of work and has a plan to fix it. The buyer can anticipate a contribution will be needed later.
If the plan is discoverable in the records of the OC, then I would suggest standing back and not saying anything beyond what is explicit in those records. It is also important for the OC or committee to avoid getting too chatty with prospective purchasers and being exposed to accusations of having misrepresented the property.
By the way, does the OC have a sinking fund plan? This is exactly the sort of thing that should be covered by a sinking fund plan. Modest contributions to the sinking fund are made every year and the plan covers known expenses such as the guttering in a few years and perhaps something else that will be expensive 10 years from now. You can also have a contingency component to cover you for unexpected repairs.
A good sinking fund plan is very equitable because it spreads the cost of repairs evenly across all owners past, present and future.