OK, so will this be sufficient to address the power drop over 75 feet?
Yes
- plan on running with additional (AC) power
Thats a Big no both the arduino and the led's are not designed to run on ac power !
Or, is 75 feet insurmountable? Is Arduino not the right device?
i think arduino can acheive this but there might be better options out there just wait and see some person with more experience can help you out 