That sounds like a recipe for Pacer mark II
Isn't the Class 230 basically that, just using second hand bodies and (thank goodness) bogies? Block the middle two doors up like LNR are doing and you have got something of a very similar size with a very similar layout.
Of course there are two other answers to the problem.
One of them is the Mk5a coach as TPE have ordered - use a D-lok now, switch to an alternative technology locomotive (or just an electric one) later.
The other is to build a more modular unit. This could either be by building a bi-mode with the diesel generation equipment and fuel tanks on removable rafts which could be swapped later for some other form of generation or batteries, or you could build a MU that will run on any modular, removable power source that simply supplies a rotating shaft of specified speed, torque and control system which again can be swapped for something else later. (Of course this, like many South East third rail EMUs, would have mountings and the low roof for a pantograph if that was what they should be converted to later).
Stadler of course already offer this kind of product - a bi-mode with a generation unit in the middle which can be converted to an EMU on-depot (one assumes) by just taking the generation unit out.
Or IOW just because it's a DMU now doesn't mean it can't be something else later if the design is sensible.