I would agree if it was a franchise based on electrification and fleet renewal, but otherwise there won’t be any electrification to use for a long time.Having bi-modes would certainly make it easier to electrify the route bit-by-bit, over a number if years.
Why would buying a bi-mode be a much bigger risk than, say, buying a DEMU?
I'm pretty sure that Stadler will build a train of whatever length you like! They have built 65m-long 3-car units and 80m-long 4-car units for GA, 65m-long 4-car units for Merseyrail as well as the 240m-long 12-car units for GA.
A bi-mode would be a bigger risk because I think they would be buying one that doesn’t exist.
The Stadlers lose length to the diesel modules - wouldn’t Chiltern have issues about required passenger length within platform length constraints?