Ok... I notice from Mr Chips profile that he is an ex-RAF engineer. The only 737 I have worked in Australia where this question came from with internal airstairs fitted is the Prime Ministers 737... granted not a 300 series... All other examples in OZ in my experience don't have the stairs fitted due to weight restrictions...
As for not touching the door handles... the reason I say this is that in the real world of commercial opps even taking paint off the aircraft is considered damage which is what will certainly occur not to mention the noise that will be heard inside the aircraft during flight which may unsettle the paying guests if the door handles are not flush after the aircraft departs...
The advice I was given is based upon many years of commercial ground opps turning 737 aircraft around at a Regional Australian Airport...
As for requiring the GPU... well... this is the part that makes me question Mr Chips engineering credentials most alarmingly... It is a very rare occasion that requires outside air for a start up with the 737.
What the hell is with the attitudes and egos that I seem to be reading in my short time checking out this forum? And so many armchair experts... I might point out that being an ex-Airforce AME qualifies one for... Zit... Zilch... Zero... in the real world of commercial ops.
To the original poster... Inbox me if you want more info on the factual requirements for 737 ground handling.