Check fuel pressure with a gauge before spending $50 on a regulator. Post the exact codes and we can help you better - intake gaskets failing typically cause irratic idle high and low for no apparent reason and could cause stalling. Intake gasket issue usually shows at least a lean condition - use the search feature to find codes usually thrown. O2 sensors from Isuzu only cost $100 ea and the Bosch aftermarket ones work fine and are around $65.

Cats will start to rattle when they are coming apart and I would definitely go with aftermarket replacements for those at less than $200 each plus a little labor to weld them in.