I would say from a commercial point of view, forget DMUs ,EMUS, ......personally I think people would only pay a premium for trophy items like locos.
So, in my view , the most popular types of loco that have been done in a mediocre fashion or could do with an upgrade.
Step forward Class 31,37,47,50,66.
Nothing really wrong with the bachy 37,47 but they could be a rolls Royce standard version. However what will the market bare for cost ? Bells and whistles I’d go to about £200.....and there’s the problem as Bachmann sound stuff already has an RRp over that.