Maybe if Donwell had had real hardship, but I would rather believe that Donwell, an important estate, had kept in the past, a few carriages; I do not imagine Mr Knightley having bought any, just kept the inherited ones — space was probably not lacking for storage. Probably, as you said, the (probably) coach used to the Coles's, as well as lighter ones. Probably not a chaise, the estate being not that enormous (Sir W. Elliot in Persuation, has one with a probably smaller estate but it ruins him, while he believes his baronetage wanted it).
Thank you for the information about oxen in Britain! It surprises me.
In France oxen were very important... and very expensive. St Vincent of Paul, the son of a well-off peasant, had his studies paid by the sale of his father's pair of oxen that he replaced by a pair of horses (less efficient).
In Sweden too, according to M. Sandwall Bergström (sorry for the childish quote), a pair of oxen was something...