I suppose a (rather expensive) option is Hornby/Lima body with Bachmann chassis....has anyone ever tried it?
you shouldn't really need to go to the length of replacing a body on a recently issued £150 model to get the windows right!!
The power unit and low friction all-wheel pick of the Bachmann BR 1st gen. DMU running gear is a very good option for a repower project. Given that some modelling activity is going to be required, and the generic similarity of the running gear of these DMU's, I would suggest that a way ahead is to pick out whatever among these classes is on best current offer. (Hattons at £72.50 for the class 108 as an example.) If you bought that 108 and used the running gear suitably modified for installation in older class 101 bodies, you would still have the class 108 bodies for sale s/h to further reduce the net spend. The lack of body spares works for you in this situation, there's got be one or two modellers out there with a 108 body damaged beyond repair.
Minor learning point on Bach DMU's, but may only apply to the class 105 model. I bought a 'power twin' 105 in order to use the unpowered running gear (with the engine and other tackle modelled on the underside) to swap with the motorised end of a regular 105 set and thus make it unpowered, as one model motor unit is more than adequate to run a permanent four car set. And what did I find? - the clip on points between body and chassis are different between powered and unpowered models of the motored car - didn't expect that.