Can't help you much with the prime except to say from my reading, if you plan on having a bus an electric fuel pump seems to be a good thing to have on hand. I would buy one now for future uses. As far as starting a bus with unknown batteries I'm pretty good at that. By all means if you can get your car near the batts that will help. I use my portable generator and a biggish battery charger to help starting in the cold, as well as ether. I have a NA 6-71 so I think the batt requirements are alot lower thatn a a turboed 6V92.
I can start my 6-71 some ether and one group 31 on a cold -10 C day if the battery is fully charged.
Don't forget that MCIs have 24V systems. Gotta have two batteries wired in series to start the bus.
The bus starter may look like a dead short to a car battery and alternator. In any event, it would take two vehicles to do the 24V jump...or one 24V bus or truck. I wouldn't want my car attached to a bus...not that it wouldn't work if the bus batteries are almost charged.
If the subject bus has been sitting so long that the batteries are dead, a borrowed set of batteries hardwired will be the most efficient way to start it..if not the only way. Dinking around with a 24V system and jumper cables could make smoke...maybe $$.
Your 6/71N probably has a higher compression ratio than a turbo 6V92TA. Still, a 6/71 may turn over easier due to the smaller displacement. Neither one will turn easily in this crap weather we're having in North Carolina...this 'global warming' is about to kill me! Alas, I digress.