Whilst buses and trains may be very different they both come under the same accessibility laws and regulations.
Whilst something like 67 and mk4s would require a lot of training and some electrical conversion etc, in terms of regulatory approval they would satisfy all the necessary regulatory requirements.
With the amount of time it’s taking to overhaul the XC HST sets (I don’t think anyone anticipated the amount of corrosion on mk3s) you do wonder would this have actually been a better option instead!
I agree regarding mk4's. It might be worth just biting the bullet and getting on with it, assuming the locos can be found.
In terms of buses, the ones around here still don't have any sort of public information system, even though that would be quite useful.