You certainly can get fan add-ons for Pis but I'd expect that to be sensible for cases where the unit is likely to work hard &/or the ambient temperature is high.
For a bit of cooling insurance, I would think that you don't need to go beyond a stick on copper or aluminium passive heat sink. An audio only use shouldn't work the Pi that hard, so it shouldn't run that hot.
Regarding Windows background processes, that's one of the reasons I'm setting up a Pi to act as an audio transport from my local NAS. The HTPC doubles as a work machine at home where we do some hard core maths for assessing investment options and this can stress the PC. I'd ultimately like to upgrade the PC but I see sense in separating functions, especially given that the incremental cost to do so is not great.
And with the Id in the picture, I've now got Engen to drive some Z-wave switches with PlayPower. On that basis, it adds to the sense of the overall plan.
The IdPi's SD card is only 16GB, so it doesn't really offer scope for onboard media storage. To access media with the IdPi as a server, you either have to opt for an external drive or a network share drive. Once the IdPi is fired up, it should only need to access the SD card for system processes that are not covered by what's loaded on to the on board RAM.
Regarding memory playback, check the pic for the options available. I've used the default on my IdPi so far and audio playback from the NAS share sounds fine to me. Even so, there are plenty of options to choose from and trial as to what suits best for your use case.