And some of them are ex-loco hauled, rather than HST spec.
Not sure that is the case with anything that is coming out of the GWR long-distance HST fleet. I believe all the conversions in 2012 or thereabouts to create more trailers for FGW sets were of redundant HST buffet cars, not former loco-hauled stock.
FGW had hoovered up lots of the ex-CrossCountry HSTs after they had been replaced by Voyagers or after use on Project Rio services between London and Manchester during the WCML modernisation work, with last influx to replace the Class 180s in 2008.
I don't know why you think it should make any difference to fitting power doors - the differences between HST and loco-hauled version are principally the electrical system (renewed anyway in the case of stock converted to HST standards) and the locations of underfloor equipment.