Two of the requirements for the Chiltern stock given at the link you gave are a reduction in ambient noise compared to the 68s and mk3s and that any diesel stock must "be compatible with alternative fuels, such as HVO, from the service introduction date."
I'm not clear whether the noise requirement would be for passengers on-board, in the driver's cab or lineside. If lineside, that seems to rule out the mark 5 stock since the Chiltern mark 3s have a genset in the DVT to allow the loco to shut down when the train is stabled. Therefore, if the concern is lineside noise there is no point replacing mark 3s with mark 5s and that requirement would seem to work in favour of 175s replacing 68s. Similarly, a 175 replacing a class 68 might be a significant noise reduction in the cab (at least when the loco is leading) (I've not been in a modern loco cab in use - but I imagine if lineside neighbours have complained it must be quite noisy for the driver).
However, if by 'ambient noise' they mean the passenger environment within the carriages the 175s probably lose because of the underfloor engines. Similarly I'm not sure if the class 175s are capable of running using HVO - are they able to do so right now or would that require mods?