Getting back to the question in hand, I suspect that Chiltern would possibly be looking for something similar to the Stadler class 755 flirt trains, where it can be used initially with diesel and AC overhead, but later converted to from diesel power to battery. The new trains would not only be replacing the MK3 plus 68, but also replacing the class 168 units.
Of Course this would be on the basis that AC overhead is put in place at some point in time on the Chiltern lines. AC overhead above LU 4th rail I am pretty sure has been done successfully before somewhere.............