The problem is with preservation that many units or locos have been so well rebuilt over the years that to preserve one as it was when we may have remembered them would be difficult. For example, of the units I used to work, if I could take one train and preserve it from this list that I used to work ("Class 50 and Mk2 coaches, classes 142, 143, 150, 153, 158, and 175") I would have to say that despite its peculiarities, it would be the class 143 every time as there is something about them to work them as a guard that made you smile. They had incredible acceleration before they had the thicker floors and the heavy airline seats. The bus type seats they had before I'd prefer. Incidently, the ex Wales & West 143's airline type seats were rebuilt from the old class 158 seats as W&W had totally redesigned and refurbished their 158 fleet which included somewhere between 20 and 40 (If I remember) new designs and improvemets.
For a guard the 143's were a joy to work as the light switch for the cab was easy to find. The cabs were spacious enough. The view of the passengers when operating the doors was relatively good and clear. It was only if one changed ends and forgot and got the doors out of sequence that were the main issue along with seating ones passengers over the five heaters and away from the doors on cold winter mornings!
Enjoying 7mm narrow gauge.