Considering that according to Ch. 14, Willoughby 'lives at an expense to which that income (700 a year) could hardly be equal' I've always wondered if perhaps Willoughby was trying to persuade Colonel Brandon to buy the brown mare because he desperately needed the cash?

The Broadview edition of S&S includes, in one of its appendices, some information on curricles from a 1796 publication.

To hanging the body of curricle with ease, every attention should be paid, and it is mostly with high, light springs, and long braces, that this is effected, with the body low between the framings of the carriage, which is built long and strong; such a curricle is the most fashionable in use.

From that I gather that if it is not hung properly in the frame, it would be an uncomfortable or rough ride. To a gentleman of the era, finding fault with how this was done would be the same as insulting someone's sports car in our day (We also have John Thorpe bragging about his curricle in Northanger Abbey). Perhaps something more is intended though? Like an implication that Marianne is in for a rough ride because Willoughby is not as careful as he should be? Or if he takes too great a risk with the curricle in the way he drives, so it has become improperly balanced--maybe he is taking risks too great in other areas?