A Bullet is perfectly capable, if prepped right. The nightmare stories I have read seem to stem from people riding ropey second hand Indian home market bikes long distances. Later export spec bikes are better made and with the right modifications are quite impressively robust. Even with the improved quality of the export market bikes there are still certain improvements that can greatly improve reliability. The main bearings can be quite suspect and it would be prudent to strip and rebuild the motor with western European made main bearings. This will also give you the opportunity to check over the rest of the motor and address the other main problem area - lubrication. There are alot of pros and cons relating to the use of high capacity oil pumps, and IMO they may not be worth the expense but there are simple modifications and checks that can be done to ensure the lubrication system is working efficiently.

The 350s certainly seem to be more robust than the 500s and, again IMO, are better suited to distance work regardless of the extra torque provided by the 500 engine. Essentially the 350 has noticeably less reciprocating mass which means less stress on the bottom end and less vibration which is of benefit to the whole bike and the rider.

Properly sorted what you end up with is a fairly robust bike that while more maintenance intensive than more modern designs is very easy to work on and pleasantly characterful. I have found spares backup in the UK at least to be far better than any other bike I have owned, and the company in question ships very quickly domestically and do also ship overseas. This company is Hitchcocks and they also have a wealth of experience relating to running and modifying post war Royal Enfields, especially the Bullet.