Camilo,
The Saudi aircraft have a 524-B2 engine which is only used in the -200.
The -500 has the B4 engines.
The big AD's that are currently due will be needed on all the Saudi aircraft which have been on the ground for several years.
Additionally bringing these aircraft up to date with EU requirements is not cheap as 411 can probably tell you !!
Bulk loading without a cargo door has not proved very successful
There are -200 aircraft out there that have cargo doors so they would be a better bet.
