Sounds like the engine is "eating" spark coils, so the most fundamental questions is, "Did you check the integrity of my engine grounds?" If the answer is no, next witness.
You may have ground gremlins in general, P0300 means random misfires on different cylinders. When a specific cylinder misfires the ECM throws a specific code, i.e., #5 will throw a P0305. If you're willing to break out a voltmeter, your grounds are something you can check very simply for yourself:
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I'll also say that changing coils onesey-twosey on an older Nissan product isn't a great idea unless you're certain the brand and part number are identical to what's already in there. Nissans from the early '00's all had feedback protocols for the coils which could cause phantom misfire codes if the parts were mismatched. So the best idea is to stick with the OE type (usually Hanshin), or if replacing with a reputable Japanese type like NGK or Denso, replace all 6.
Vis the alternator, if the car doesn't have starting problems and there's no generator light on the dash, the only right question is, "Why do you say it's bad?" It's possible it's overcharging, but that's also something you can easily check yourself. Your '03 doesn't have a "smart" charging system, so you should read about 14.0V~14.8V across the battery terminals with the engine running (nominal is 14.2V for most Nissan units). It should stay in that range when loaded by the blower and high beams. If so, there's nothing wrong with it electrically, full stop.