re: option #3: It's my understanding that McIntosh used enterprise grade Seagate Hard drives in the MS750, not sure about the MS300 though...
"the 750GB hard drive, which McIntosh's product manager, Ron Cornelius, described as a mission-critical Seagate."
These hd's are designed to be on 7x24x365 in data centers. You might think data centers are very cold, but usually any device with many (>10) hard drives, such as a raid or san, get hot, even with massive hvac handlers forcing cold air through the dc. Designed to function in this type of environment, these drives have a mtbf of 1.2 million hours. The Seagate ST31000340NS is the 1TB sata model if you want to read up on the technical details.
I personally run equipment in multiple data centers, and I would speculate that your hd will be fine if you disable the fan, though it's obviously not optimal. That's assuming your room is not generally hot, say during a hot summer day, or if you have other hot equipment around this device without reasonable ventilation. Simply running the a/c at 75F would probably keep your system happy or maybe you can only use it during cooler hours and can leave it powered off on hot summer days.
That being said, your hd will be happier with the fan functioning properly, and mtbf is a statistic. Also, I've only presented information on the hard drive itself, not the components surrounding it within the MS300. Just trying to provide more information and options you can factor into your decision...
Cheers, Steve