Stock alternator up to the task? If not, how many Amps will I need?

The alternator's job is to keep the battery charged, say over a week of driving. The battery can meet the peak fan load by itself, and the fans shouldn't be running most of the time. So, answer depends on where you live and how much time you spend at stoplights in the summer so the fans kick on. I would first try your current alternator. Put the battery on a charger after a day, then week. If it takes little time to show "full charge", then your alternator is keeping up. You will power the fans via a relay, so current stays in the engine compartment and won't stress your bulkhead and ammeter any more than currently. Realize that your dash ammeter won't show the current consumed by the fans, just as it doesn't show current used by other under-hood loads (starter, horn, add-ons), so with fans running the ammeter might show "C" when actually the battery is discharging.