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

If you tap power from BATT+, the ammeter will not measure the draw. The dash ammeter measures current on the BATT+ wire under-dash, so if you tap it anywhere after the ammeter, the current draw will not be measured. In the factory design, currents by the starter, horn, or optional spotlight (police option) are not indicated on the ammeter. Mine seems to be a minority opinion. Others seem to insist that the alternator meet all max instantaneous loads when idling. If you do install a mega-alternator, understand that you may fry your factory wiring and dash ammeter, unless you perform the "MAD Bypass" (many posts).