Have to say I am biased toward the Thema as well. I have had 14 or so of them, with one solitary Dedra, a 1993 1.8LE for a week! Compared to modern cars the Thema is tiny. With regard to the folding rear seats, the Series 1 and 2 had folding seats, only the 8.32 and S3 versions didn't. I have not driven a Dedra turbo so am not qualified to comment on it particularly.
What I would say is the 8v engine is a far better bet than the 16v, in terms of longevity. So that rules out most Themas, unless you find an S1 V6.
I thoroughly agree with the point being made regarding costs of running a Dedra. As long as you get a reasonably sound one, then it should prove less costly to run than a modern, because of the, frankly stupid and disgraceful, cost of modern car spares. And as has been said already, too many electronics.
So, although on first thoughts, running a Dedra as a daily is not for the faint hearted, there are many good arguments for doing so. And don't forget, Lancias (as with all cars) run far better if used regularly.
I would be inclined to go for it really!