One of the providers offered to come to the property for an informal conversation with the architect.
That does not sound to me like an architect in the literal sense of a freelance architect, i.e. a planner who understands the client; rather like a building application draftsman. This means, not least: someone who has nothing to do with the discretion of pipe installation, ensuring that switches are not poorly fitted, or even worse: with creating residential comfort, because none of that is relevant for approval.
So I fear nothing more than a sales talk, in which the provider’s psychologically most important reinforcement is having the relevant property "with them," and showing the face of their draftsman for further support, so that the association with professional competence can be established. I expect no more than a shared visual impression of the dimensions and flatness (or slope) of the building plot.
You will express your thoughts about your building dream, the provider will say that it is feasible, and the draftsman will nod competently. On the one hand, the situation will be far from testing the "architect" to see whether he is a good or very good one—for this job one usually accepts satisfactory to sufficient.
And on the other hand—actually the most important: from my point of view, you should prepare yourself less with questions and more with answers. Look at the development plan and ask yourself whether you understand it: for example, number of storeys, building heights and their reference points in the terrain can make two properties that look identical buildable to a very different degree.
You best assess the planner’s and implementer’s qualities of the general contractor and their team by checking their references. That means also: you better look at their "architect" on one of their current construction sites and have them explain which special features they had to consider there.